The PCconfig thread contains quite a few tasks. Below I've pasted a list of some of the tasks that run in that thread.
Command line interface
Maintenence events for various interfaces (Ethernet, ADSL, SHDSL, DDS, etc.)
EFM (Ethernet in first mile)
NQM Network Quality Monitor
Bridging (Aging cache)
VoIP Name Service
VQM (Voice Quality Monitoring)
Maintenence event for layer 2 (encap) protocols (PPP, FR, ATM)
Were you viewing the CPU in the web interface? Sometimes the web interface has a tendency to make the CPU spike for a short period.
Let us know if you have any questions..