PC question: What's wrong with my computer, the hard drive or the operating system?

This has stumped me for a while now, so perhaps the Collective will be able to give me some insight about this issue.

My PC, which was home built in 2005, running Windows XP Professional. It has been repeatedly updated with new hardware since. In February of this year the computer totally crashed and I replaced my main hard drive as well as my motherboard and processor. Now, only a few months later, my computer has failed yet again in an identical fashion as the last time. I’m unable to start my computer; it either freezes at the Windows loading screen or gives me an error that a system file is corrupt and it cannot start up. Occasionally it gets as far as being able to do a disk check; sometimes it finds errors, sometimes it doesn’t. Still, it’s only once in a blue moon that I’m able to successfully start up the computer, in which case it seems to work for a while, albeit rather slowly.

Another odd occurrence is that my computer seems to be unable to recognize my USB keyboard or mouse until the computer has fully started up. Each time it gives me the notification that new hardware has been found and re-installs the hardware with each restart. I have switched to an older, non-USB keyboard just to be able to press keys on the start-up screen.

The only things that have been the same between the two recent crashes has been the OS and the game The Sims 3 – I believe my copy of the expansion pack, World Adventures, was corrupt, and I am getting a replacement disk from EA games. I have also tried to reformat my hard drive, but the computer freezes during the early stages of the reformatting process, making it impossible.

My dad thinks the hard drive has died, but it’s only a few months old. I personally suspect Windows XP is the problem and I’m considering upgrading to Windows 7.

I know it’s a complicated problem, but does anyone have any insight or experience with a crash like this? Anything, any suggestions, would be greatly appreciated.