Thomas,
Puzzling! Are there any entries in the event log (run eventvwr.msc) which might cast some light? Have you tried with machine disconnected from network (unplug cable)? What about with NIC removed? (must admit I'm not sure if it would let you then).
You could try a repair reinstall (keeps data, apps, settings, loses windows updates - need to reapply service packs - unless you've an install disk with latest included (slipstreamed) & windows updates). To do that, boot from 2k install Cd, choose new install (not repair). It should find your installation, and offer to repair it (if it doesn't, abandon install) - accept this (type R). Then mainly like normal install (no guarantees it would fix this, but....)