Definitely not disk space, RAM, or too much multitasking, but it could be shell extensions (don't think I have anything unusual) or virtual network adapters (VirtualBox, Evolve, and Viscocity). Never gotten any clues out of the performance tab, but I didn't know about Process Explorer until a few months ago. I'll definitely take a deeper poke at it if it shows up again.

At least the days of LAN parties invariably starting with someone wiping their computer to fix a networking problem are done with!