As suggested..use the stinger..it's a wicked freeware removal tool.
As for prevention tho.. as usual.. make sure you've always run "windows update" and installed all latest critical patches. (SP2 for Win XP isn't too bad if you turn of DEP!). Also.. something like Ad-Aware (Se Pro version) is handy as the "Ad-Watch" side of things helps prevent trojans from being installed by restricting the changing of the registry / popups / activex components etc..

or just don't visit any dodgy sites