Possible cpu failure

hello, its my first post here. I have been having problems with my computer and hope some one can help me.
specs

Msi kt880 delta mother board
Amd athlon 3000+ cpu
Sapphire 3650 hd 512 mb graphics card agp slot
maxtor hard drive
and 2X1 gb dual channel ddr ram kingston
power supply unknown 480 watts
windows xp

I got the graphics card and ram for christmas and installed the parts, in hopes of being able to play call of duty4 and other games. When I go to open up call of duty 4 or any other program that creates a decent amount of load my computer freezes, The cpu is up at 100% usage and the hard drive light is stuck on. I tried running benchmark 05 multiple times and the computer freezes before it really even starts to load anything. It will freeze pretty much before and sound or graphics really load.

Before Installed the graphics card and ram I have always had issues with my cpu very easily being ran at 100% usage. For example if I try to stream video off of say abc.com then my computer would lag and cpu would run at 100%. I figured that it was due to my low amount of ram I was running and that was 756mb. Well after installing ram it still runs at 100% usage. The processor is running at 2.1 ghz according to corecell.
Other weird things
For some reason while the computer is under load my cpu clock jumps all over the place Ive watched it jump to 3.0 ghz before. While the cpu clock speed jumps all over the place i also notice the varies voltages are jumping around to. I have not over clocked the cpu and dynamic overclocking is disabled. Also cpu temp might be a little bit on the hot side about 45 c at idle and up to 60 c under load. The hard drive light seems to stay on alot.

I have tried reformatting hd, reinstalling games and programs, went back to the old ram sticks, reinstalled drivers. no difference. Also when I built the computer roughly 4 or 5 years ago my brother said that I might have problems with the power supply as it was his before and he said he wasnt sure if it was working right.
So my guess is that its a hardware problem either with the power supply, hard drive or cpu.
I would think that my computer would be able to run call of duty4 ok. Any help would be greatly appreciated, as I have put about 30+ hours into this problem

Eventually I will build a new pc but right now I dont have cash for it. Any other ideas?