Hijackthis is good for getting the details on a worm or virus when you really know something is wrong but it's rather a lot of trouble when you don't know you've got a problem. You could just the the 'Power Toys' and 'Kernel Toys' from Microsoft:
http://www.microsoft.com/windows95/downloads/
And use 'Wintop' from the Kernel Toys to check your running processes. This displays CPU usage, too, so it would tell you where your CPU power is going. It's originally for Win95/98 but as far as I know this works under Win NT/2000/XP as well.