My computer may be hijacked

If my DSL modem is on, CPU usage spikes every five seconds.  When not connected it goes to 40-60%.  When connected it goes to 60-100%.

McAfee, Webroot, Malwarebytes, HijackThis all can find nothing.  A helper in the Msoft forum said "Cut your losses and format".  I will have to format my hard drive or buy a new computer.  I will not buy a new computer while Vista is the only OS available.

Where can I find complete instructions for formatting my hard drive and installing XP with all Service Packs and loading my data without bringing back in the malware that is killing my system?

I'm sorry if this is not formatted but Preview shows me nothing at all.