You could check whether the problem is software related by starting in Safe Mode. If the computer runs ok then you need to see what program is causing the problem. Do you have a hardware monitoring program? The computer could be shutting down as a protection against voltage or temperature being out of limits.
Why do you think you need a new hard drive? At the very worst, you can reformat your drive and reinstall Windows and your software. Have you tried scanning for virusses. Most of the major antivirus companies will do an online scan for you. You can find these by doing a Google search for "online virus scan", I found Norton (Symantec), Trend and Kapersky on the first page, and I'm sure MsAfee will also do a scan.
Re. your hard drive being half full, are you saying that you don't know what is filling it?
Panda, download panda software free online virus scan then download stinger and destroy the virus it sounds exactly like the blaster virus, you can also try microsoft;s site and download a patch to isolate and repair your problems... good luck