Windows corruption or virus, your pick. Does it do it in safemode also?
May want to run a memory test like memtest86plus to make sure your new memory isnt the problem. get the plus version as they have bootable CD images.
If its a windows corruption, see if you can do a "SFC /scannow" from the start/run prompt. Preferably in safemode. Have your WinXP install CD handy.
But I would be paranoid of a virus first.
http://kaspersky.com/ and
http://www.pandasoftware.com/ have free online scans. Both are quick to update their database with the latest virus. McAfee and Norton can take 3 to 7 days to update.
For those really pesky viruses, scan your system with rootkit revealer and see what is purposely hidding on your computer from windows.
http://www.sysinternals.com/Utilities/R ... ealer.html
After thats all ruled out, may want to do a harddrive error checking scan to make sure the drive aint flaking.