Depending on how the error message came on, it could have been legitimate. Then again, it might have been a virus or trojan that was telling you falsely that it was corrupt.
You can also do a SFC /SCANNOW
. Make certain you have the original CD in the drive, as it may ask for it. HOWEVER, if you can boot up, like you said, this may not help.
The SFC command checks all system files and if they have been corrupted, it attempts to fix it.
You can also test your memory. Go http://www.memtest86.com/
and you can download a free memory tester. You will need to burn it to a blank CD. Put it in your drive and boot up the computer. It will load the test and start right in.
Rule out these 2 things first. Then you can check some other things.