How much memory do you have? (Edit: Also, what how much virtual memory do you have configured?) Are you overclocking at all? You ought to run memtest86 to do a full test on your memory.
http://www.memtest86.com/download.html
I would get the ISO and burn it, if you can, it makes a bootable disk.
This is just to make sure your memory subsystem is OK. I once had my mem clock speeds wrong, and memtest86 got to a certain point and the machine went BSOD (it must have overheated). It was new memory, so I went through all the settings, and found one wrong. Then I ran memtest86 again, and it was OK.