My 5 1/2 year old XPS 400 had the same problem.  I tried everything on every forum I could find and nothing worked.  After much experimenting HERE IS WHAT FINALLY WORKED FOR ME:  Put the Dell OS CD into an external USB CD or DVD drive.  The computer will recognize that.  For some reason a lot of Dell computers won't recognize the internal IDE controlled DVD drives when the hard disk is replaced but, it will recognize an external drive.  I bought a Vantec SATA/IDE to USB 2/0 Adapter for $18 plus shipping from TigerDirect and used an old extra internal DVD drive for my new external drive for the OS install.  Everything worked without a hitch.  I fully installed my OS yesterday from the old Dell OS Re-installation CD.  Now my old Dell computer has new life and  runs great!  BTW, the same SATA/IDE to USB Adapter has allowed me to off load the data from the old drive onto the new (my problem was there was a hard disk error in the system file so the computer wouldn't boot up but, the data was still good).
The hard drive on my dimension e510 died, so I just installed a new one. My problem is that I can't get my Dell OS CD for Windows XP to boot. I changed the boot sequence to boot from the CD drive. I can get the resource CD to boot, (so i know it's not a problem with my CD drive) but it will only run the diagnostic testing. When I put the OS CD in, it says "selected boot device not available". Any suggestions? Thanks in advance, Kim
can't boot from my dell restore cd