It could be the HD or a memory problem, but what I would do first, is run a comprehensive anti-virus scan. You need to boot to a rescue disc, such as the one here
. To use it, burn the file to a dvd, then boot from the dvd instead of your hard drive. This is a Linux based program, but doesn't need any special knowledge to run it. It will automatically d/l the latest virus definitions, if you have it plugged in to your modem or router. (it doesn't seem to locate the wireless adapter on my rig).
This bypasses Windows, and everything on the hard drive, and will scan very thoroughly. There is also a HD diagnostic program which should tell you if the HD is not in good shape.