Hello again, Sorry for the delay in updating this thread. I ran the GMER rootkit scanner as you suggested, and it came up negative.  I ran it first in its verbose mode (Show All checked - the log file produced was huge, so I have not attached it).  I then ran it a second time with "Show All" unchecked (to see if it would produce a more manageable log), but it crashed with a BSOD when windows update decided to kick in.   After a restart, I let the windows updates run their course, and following the next restart (with windows updates installed) the previously observed conflict (mcafee vs winlogon) went away completely.  It seems that one of the recent critical updates to XP has solved the problem, leading me to believe there may have been a problem with an earlier windows update. As the box has been running now for several days without issue, I think we can close out this thread now.  Thanks for your help.
Very recently, this Dell PC running XP Home (sp3, with all updates current) and McAfee (VirusScan build 15.0.288. engine version 5400.1158, DAT version 6375 of 6/13/2011) begain experiencing very high CPU spikes and slow performance.  The winlogon.exe process was spiking from 10% to 80% about once every second, and web searches suggested this had in the past been a symptom of a malware infection.  McAfee full scans revealed nothing. However, the CPU activity would return to normal when McAfee real-time scanning is turned off, and will spike again when real-time protection is re-enabled.  McAfee's only suggestion was to reinstall McAfee (done, problem persists) so it's time to explore other options. Thanks in advance for your help. HIJACKTHIS LOG FOLLOWS: Logfile of Trend Micro HijackThis v2.0.4Scan saved at 8:16:10 PM, on 6/14/2011Platform: Windows XP SP3 (WinNT 5.01.2600)MSIE: Internet Explorer v8.00 (8.00.6001.18702)Boot mode: Normal Running processes:C:\WINDOWS\System32\smss.exeC:\WINDOWS\system32\winlogon.exeC:\WINDOWS\system32\services.exeC:\WINDOWS\system32\lsass.exeC:\WINDOWS\system32\svchost.exeC:\WINDOWS\System32\svchost.exeC:\WINDOWS\system32\spoolsv.exeC:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\mscorsvw.exeC:\Program Files\Java\jre6\bin\jqs.exeC:\Program Files\Common Files\McAfee\McSvcHost\McSvHost.exeC:\Program Files\Common Files\McAfee\SystemCore\mfevtps.exeC:\WINDOWS\System32\svchost.exeC:\Program Files\Common Files\McAfee\SystemCore\mcshield.exeC:\Program Files\Common Files\McAfee\SystemCore\mfefire.exeC:\WINDOWS\Explorer.EXEC:\Program Files\McAfee.com\Agent\mcagent.exeC:\WINDOWS\system32\ctfmon.exeC:\WINDOWS\system32\taskmgr.exeC:\Program Files\Internet Explorer\iexplore.exeC:\Program Files\Internet Explorer\iexplore.exeC:\Program Files\SiteAdvisor\6253\SiteAdv.exeC:\Program Files\Internet Explorer\iexplore.exeC:\Documents and Settings\USER02\Desktop\HijackThis.exe R1 - HKCU\Software\Microsoft\Internet Explorer\Main,Default_Page_URL = [URL] - HKCU\Software\Microsoft\Internet Explorer\Main,Start Page = [URL] - HKLM\Software\Microsoft\Internet Explorer\Main,Default_Page_URL = [URL] - HKLM\Software\Microsoft\Internet Explorer\Main,Search Bar = [URL] - HKLM\Software\Microsoft\Internet Explorer\Main,Start Page = [URL] - HKCU\Software\Microsoft\Internet Explorer\Main,Window Title = Windows Internet Explorer provided by Yahoo!R3 - URLSearchHook: (no name) - {EF99BD32-C1FB-11D2-892F-0090271D4F88} - (no file)O2 - BHO: (no name) - {02478D38-C3F9-4EFB-9B51-7695ECA05670} - (no file)O2 - BHO: Adobe PDF Reader Link Helper - {06849E9F-C8D7-4D59-B87D-784B7D6BE0B3} - C:\Program Files\Adobe\Acrobat 7.0\ActiveX\AcroIEHelper.dll (file missing)O2 - BHO: (no name) - {089FD14D-132B-48FC-8861-0048AE113215} - C:\Program Files\SiteAdvisor\6253\SiteAdv.dllO2 - BHO: McAfee Phishing Filter - {27B4851A-3207-45A2-B947-BE8AFE6163AB} - c:\progra~1\mcafee\msk\mskapbho.dll (file missing)O2 - BHO: scriptproxy - {7DB2D5A0-7241-4E79-B68D-6309F01C5231} - C:\Program Files\Common Files\McAfee\SystemCore\ScriptSn.20110531175322.dllO2 - BHO: Windows Live Sign-in Helper - {9030D464-4C02-4ABF-8ECC-5164760863C6} - C:\Program Files\Common Files\Microsoft Shared\Windows Live\WindowsLiveLogin.dllO2 - BHO: Java(tm) Plug-In 2 SSV Helper - {DBC80044-A445-435b-BC74-9C25C1C588A9} - C:\Program Files\Java\jre6\bin\jp2ssv.dllO2 - BHO: JQSIEStartDetectorImpl - {E7E6F031-17CE-4C07-BC86-EABFE594F69C} - C:\Program Files\Java\jre6\lib\deploy\jqs\ie\jqs_plugin.dllO3 - Toolbar: McAfee SiteAdvisor - {0BF43445-2F28-4351-9252-17FE6E806AA0} - C:\Program Files\SiteAdvisor\6253\SiteAdv.dllO4 - HKLM\..\Run: [IgfxTray] C:\WINDOWS\system32\igfxtray.exeO4 - HKLM\..\Run: [Lexmark 5200 series] "C:\Program Files\Lexmark 5200 series\lxbtbmgr.exe"O4 - HKLM\..\Run: [LXBTCATS] rundll32 C:\WINDOWS\System32\spool\DRIVERS\W32X86\3\LXBTtime.dll,_RunDLLEntry[NAME] - HKLM\..\Run: [LVCOMS] C:\Program Files\Common Files\Logitech\QCDriver2\LVCOMS.EXEO4 - HKLM\..\Run: [UserFaultCheck] %systemroot%\system32\dumprep 0 -uO4 - HKLM\..\Run: [SiteAdvisor] C:\Program Files\SiteAdvisor\6172\SiteAdv.exeO4 - HKLM\..\Run: [REGSHAVE] C:\Program Files\REGSHAVE\REGSHAVE.EXE /AUTORUNO4 - HKLM\..\Run: [mcui_exe] "C:\Program Files\McAfee.com\Agent\mcagent.exe" /runkeyO4 - HKLM\..\Run: [SunJavaUpdateSched] "C:\Program Files\Common Files\Java\Java Update\jusched.exe"O4 - HKCU\..\Run: [MsnMsgr] "C:\Program Files\MSN Messenger\MsnMsgr.Exe" /backgroundO4 - HKCU\..\Run: [DellSupportCenter] "C:\Program Files\Dell Support Center\bin\sprtcmd.exe" /P DellSupportCenterO4 - HKCU\..\Run: [ctfmon.exe] C:\WINDOWS\system32\ctfmon.exeO4 - HKCU\..\Run: [Search Protection] C:\Program Files\Yahoo!\Search Protection\SearchProtection.exeO9 - Extra button: (no name) - {08B0E5C0-4FCB-11CF-AAA5-00401C608501} - C:\Program Files\Java\jre6\bin\jp2iexp.dllO9 - Extra 'Tools' menuitem: Sun Java Console - {08B0E5C0-4FCB-11CF-AAA5-00401C608501} - C:\Program Files\Java\jre6\bin\jp2iexp.dllO9 - Extra button: (no name) - {e2e2dd38-d088-4134-82b7-f2ba38496583} - C:\WINDOWS\Network Diagnostic\xpnetdiag.exeO9 - Extra 'Tools' menuitem: [NAME].dll,-20001 - {e2e2dd38-d088-4134-82b7-f2ba38496583} - C:\WINDOWS\Network Diagnostic\xpnetdiag.exeO9 - Extra button: Messenger - {FB5F1910-F110-11d2-BB9E-00C04F795683} - C:\Program Files\Messenger\msmsgs.exeO9 - Extra 'Tools' menuitem: Windows Messenger - {FB5F1910-F110-11d2-BB9E-00C04F795683} - C:\Program Files\Messenger\msmsgs.exeO16 - DPF: {4ED9DDF0-7479-4BBE-9335-5A1EDB1D8A21} (McAfee.com Operating System Class) - [URL] - DPF: {BCC0FF27-31D9-4614-A68E-C18E1ADA4389} (DwnldGroupMgr Class) - [URL] - DPF: {C1FDEE68-98D5-4F42-A4DD-D0BECF5077EB} (EPUImageControl Class) - [URL] - Filter: application/x-mfe-ipt - {3EF5086B-5478-4598-A054-786C45D75692} - c:\progra~1\mcafee\msc\mcsniepl.dllO22 - SharedTaskScheduler: Browseui preloader - {438755C2-A8BA-11D1-B96B-00A0C90312E1} - C:\WINDOWS\System32\browseui.dllO22 - SharedTaskScheduler: Component Categories cache daemon - {8C7461EF-2B13-11d2-BE35-3078302C2030} - C:\WINDOWS\System32\browseui.dllO23 - Service: Java Quick Starter (JavaQuickStarterService) - Sun Microsystems, Inc. - C:\Program Files\Java\jre6\bin\jqs.exeO23 - Service: lxbt_device - Lexmark International, Inc. - C:\WINDOWS\system32\lxbtcoms.exeO23 - Service: McAfee Personal Firewall Service (McMPFSvc) - McAfee, Inc. - C:\Program Files\Common Files\Mcafee\McSvcHost\McSvHost.exeO23 - Service: McAfee Services (mcmscsvc) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\McSvcHost\McSvHost.exeO23 - Service: McAfee VirusScan Announcer (McNaiAnn) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\McSvcHost\McSvHost.exeO23 - Service: McAfee Network Agent (McNASvc) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\McSvcHost\McSvHost.exeO23 - Service: McAfee Scanner (McODS) - McAfee, Inc. - C:\Program Files\McAfee\VirusScan\mcods.exeO23 - Service: McAfee Proxy Service (McProxy) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\McSvcHost\McSvHost.exeO23 - Service: McAfee McShield (McShield) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\SystemCore\\mcshield.exeO23 - Service: McAfee Firewall Core Service (mfefire) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\SystemCore\\mfefire.exeO23 - Service: McAfee Validation Trust Protection Service (mfevtp) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\SystemCore\mfevtps.exeO23 - Service: McAfee Anti-Spam Service (MSK80Service) - McAfee, Inc. - C:\Program Files\Common Files\Mcafee\McSvcHost\McSvHost.exe --End of file - 7432 bytes  
Winlogon.exe process thrashing with McAfee real-time scanning - malware 
infection?