Have you run any tests on it?
You can use memtest downloadable from here:
Memtest86 - Download Page
to test the memory. Leave it running for at least a good few hours, errors can come up every few passes, so even if it says 100% through, it will carry on and errors can still be found.
You can also test your hard drive, just get the utility from your hard drive manufacturer's website and boot with that and run the tests