My 1.7GHz procesor is at 100%, and no aplications are running!! Why?

[TIBI3350]

I have a AMD 1.7GHz procesor and after I start my computer (on XP) it's working at 100%. If I unplug my network cable it work normaly? What can I do?

 

[Lorand]

It could be a worm that sends emails from your computer. First of all check out what process or program uses intensely the cpu with Procexp (http://www.sysinternals.com/files/procexpnt.zip).

 

[Praetor]

after I start my computer (on XP) it's working at 100%

Any processes in particular using the CPU?