First, I would try spybot as well. There's something not right on there. Anti-virus is nice, but it often causes systems to act like yours. My first opinion is that the problem is with the software. I would track down your XP disc, back up your data and do a repair install. That should set your system right. Perhaps first uninstall software you don't use.

If it's a hardware problem, it's not likely your hard drive, but most of the manufacturers have programs you can use to check the drives. There are also free programs out there to check your memory. Most hardware issues would leave you with OS crashes or the inability to boot.

I would also suggest you add as much memory to the machine as you can once fixed. It will work much better with 4GB of ram than it will 1GB and memory is cheap right now. A rule of thumb is that unless you do gaming, graphic design, professional work, etc, then memory is much more important than processor speed.