CPU running too high....

I am noticing my CPU is averaging 50% right now according to Task Manager. What is bugging me is nothing is showing up why.... I've gone thru the usuals, uninstalled anything that wasnt being used, disabled all services that I dont need, I checked what processes are running with TM and also a process script, checked to see if anything was running at start up, including in the registry, ran a port monitor, a packet sniffer & netstat -n - nothing, did a complete anti-virus scan - nothing, I have hardware firewall that is intact.... crap, aside from reformatting I am at a lost.... Anything - Malevolent? Mattboy? Oh I also ran AntiSpy.exe and nothing came up. I only install applications that are known to be good (if they are free) and with the firewall and anti virus, run across the occasional virus that gets caught in an email. I did a re install about a month 1/2 ago so it is still fresh and there is lots of room and ram available. Driving me nuts. Nobody seems to have a clue why.

Just for future reference incase someone else has this problem. There is a .dll file (I think it was msafd.dll) that has been known to cause way too many requests to the CPU, which in turn can cause up to 100% CPU usage. I did a few updates and Voila - back to normal. I am not 100% sure which update it was. This would be an issue for windows 2k users, pre SP3. Although I had installed SP3, but I checked the version # of that file after finding some reference to it on the MS database, and it was old.