I tried reinstalling the video cards but to no avail. Finally, I formatted the hard drive, which unfortunately cost me the data on the drive, of course, but I was able to reinstall the OS. Everything seems to be fine now. Any suggestions as to what might have gone wrong in the first place? While using the windows repair tool I got messages stating the boot sector was damaged and also a message stating something to the effect of some parts of the drive not being recoverable.