Need Help: One core maxing itself out

Hi all, this is my first post here. Usually when I've got a problem I can search here and Google and find a fix, but I haven't been able to find this one anywhere.

Recently, I noticed one of my CPU cores running at 100%, while the other one is running at 5-10%, with no processes taking up that much CPU power. Nothing seems to fix the problem while the computer is running, but if I restart the computer, it goes back to normal for a while.

Computer stats:

HP media center pc m7760n
core2 6400 @ 2.13
2gigs of ram
250 gig hdd x2
Microsoft Windows Vista 32 bit service pk 2

Here is the link to the hp website about this computer:
http://h10025.www1.hp.com/ewfrf/wc [...] g=en&cc=ca

I've made sure Windows is not indexing, run Avast, Spybot and Malwarebytes (all are updated), and taken the computer off of my network.

I ran a MRI disc, and it stalls out while checking the USB ports. All other diagnostic tests pass, but it will not complete the USB one. All USB ports are functional, though. The MRI disc is the diagnostic disc that Geek Squad uses.

I've seen a few examples of this problem mentioned online, but what I've seen seems to all be a problem from startup. Restarting the computer always brings the CPU back to normal, but one core consistently maxes itself out after several hours, even without having run any programs on the computer.

Any suggestions would be very much appreciated, and if you need more info just ask and I will do my best to answer as I am not a computer expert.
Thanks for your time

ps If this is in the wrong forum please move it.

More than likely that the process taking up your processor time is not optimised for multi core CPU's

Ctrl *** Esc? I just learned something new. =)

Thanks for the suggestions, everyone.

Apparently, computers are like cars, and refuse to fail when you want them to. Once it does again, I'll be able to check for sure which processes are at the top, but from what I remember, it was System Idle Process at the top in the 90%s, followed by dwm.exe and taskmgr.exe, both under 5% apiece.

shadow and wa1: I've already run Avast!, is there another (free) antivirus software you would suggest?

ChrisCornell: That's one of the parts that's confusing me so much. The processes list says that the processors are 90% idle, but the graph shows that one processor is 100% busy.

Download and run process explorer; I've seen something like this before where the load was caused by hardware interrupts (usually mouse, keyboard inputs) but the system was constantly using at least 45% of the cpu. The best solution I found was to reinstall the OS.

Don't reinstall until you know for sure, though, as it might be fixable without the reinstall.

You could try AVG or Avira (Personal Edition which is free: http://www.avira.com/en/download/index.html )