svchost.exe takes all cpu time

Lately when I start my PC it seems to take an excessive time to get going.
When I check the TaskManager I notice one instance of svchost.exe using
almost 100% CPU time. I've run my virus checker AVG and spyware checker
Ad-Aware SE Personal but no problems found.

From another message I saw that svchost.exe can be used by trojans and the
legit copy is in the windows\system32 folder. I did a search and found it
in 2 other locations - windows\ServicePackFiles\i386 and
windows\$NtservicePackUninstall$ folders.

I'm running XP Home. Any suggestions other than just wait for the PC to
wake up (it is getting old).