Some other things to consider; a bad stick of RAM can do things like this, if you're doing something complex a low-powered or bad power supply can create voltage fluctuations and cause problems, not enough memory, or scratch disc space, and as I have said before, delete your temp files regularly. Every time you install a program or it crashes it leaves behind a temp file that can cause havoc.
Computers are like people, they have a personality based on the hardware and software installed and they get cranky as they get older...