One nagging issue is some driver could be defective and you only sort that out by trial and error. I don't see in your post you have the latest service pack, a firewall and scanned the machine with Housecall.
Have a wierd problem with this laptop that I've had since new (about 5yrs old, running win2k), which has only recently cropped up. Every time I go online (dialup), after about 5min the processor peaks at 100% usage and the system slows to almost a halt. Even after I get off-line, the processor continues this way until I reboot.
I ran updated virus (AntiVir) and Spyware (AdAware and Spybot) get nothing beyond a few tracking cookies. It's a 750mz Celeron w/128 mem. I run Hijack This to check runnning processes -- nothing out of the ordinary.
I do notice that the SVCHOST.EXE file seems to hog up all the processor resources during this time...