Ha...this was my EXACT situation last week.
It's Virex! Something about it and Tiger don't play nice, I suppose. In fact, Apple no longer offers it on .Mac (not sure if that'll change in the future or what).
But someone here sent me a link that completely removes Virex and all its stuff from your Mac and I haven't heard the fan since. And yes, in my Activity Monitor, it was showing that vshield thing taking ALL my resources...my fan was churning and my PowerBook was hot as Pop-Tart.
Here's the link I used to uninstall Virex...
http://www.networkassociates.com/us/...ex/virex76.htm