CMOs battery and clear CMOs are simple inexpensive steps to try. No promise they will fix issue.  Win 7 iso download link burn iso to a dvd.  After successful install you need a genuine Win 7 product key to activate it.  Note 7 is not secure on internet. You are vulnerable as soon as you surf internet.
I have a VOSTRO 260 PC that sometimes freezes when I am logged out or have the screen locked.  Online support suggests updating the BIOS as a first step to fix the problem.  The Dell support site lists MS-A08.exe as the program to update the BIOS on my system.  However, after responding 'yes' to the prompt for permission to make changes to my computer, it does not run. Does anyone have any suggestions?  
Updating BIOS