Hey guys I'll post what i put up on Yahoo Answers:
The problem started some time ago, originally I thought I didn't have enough ram or I needed a faster graphics card but that didn't fix it. Here are my computer specs:
1) Pentium 4 2.0 Ghz
2) 1.5 Gig Ram
3) 80 Gig HD
4) Windows XP Sp2
5) Nvidia GeForce 6600 GT
I suspect the problem is due to some program I installed. The symptoms are that programs and processes on the computer signficantly slow down WITHOUT a reduction in Ram usage but the CPU is running at nearly 100% usage... this is exemplified when I try to run any video media file where the video is choppy. I usually do a lot of 3d animation, using Blender in particular... but I've tried uninstalling it but it doesn't make any difference. I have done the following in summary:
1) Virus and spyware check
2) Registry clean and defragmentation (used several programs that not only clean the registry but clean up all the junk files lying around the hardrive)
3) Defragmented the HD
None of these have worked.
Just a few more details...
I have done pretty much everything that has been suggested thus far... i don't have any games installed so it won't be from that and I also don't have a screensaver.
FYI - System Restore does fix the problem (that's how I used to fix it) but unfortunately I don't have any restore points to go back to, where the computer was running normally... so now I have to fix the problem.
When I did do a system restore the problem went away.. then I setup a scene render overnight using Blender and when I woke up in them morning I also found that AVG decided to run a virus checker at the same time, but my rendered avi finished and I stopped the antivirus scan. After that when I tried to view any media file, the video would be choppy and the general processor speed hit the floor... everything is take two or three times longer than it used to.
My last resort option is to reinstall XP and that will probably fix the problem... but if I can do anything to avoid that it would be awesome.