So here's my deal. Every 10min or so, sometimes more and sometimes less, SVCHost (network service user) will take all of my processor time for like half a minute. After that lag everything goes back to normal until it happens again. It doesn't take more memory, just 5mb all the time, and it doesn't seem to be related to network usage.
The only thing I've changed recently is I switched from wireless to wired LAN. The wireless connection was only a few months old, and before it I had a wired connection with no problems like this.

Just a tip: Try disabling the Windows Update Service
It was ♥♥♥♥ things up for me

(start / run / services.msc)
I'm assuming WinXP

If you are using Windows Vista go to Start > Control Panel > Security > Security Center > Turn Automatic Update On or Off (the little blue link under Windows Update).