It all depends on the type of virus that you have.
There are several tools available to help you though...
go to http://www.mcafee.com and download the free stinger removal tool. That will rid of you of many problems.....
Then hit www.lavasoft.com for adaware to remove the ad ware components.....
Then search for hijack this spyware removal tool. That will take care of browser issues and programs that are in startup and the registry that shouldnt be there. .....
Then last but not least search for Bazooka Spyware Removal Tool. Bazooka will scan and search for other spyware, trojans, and ad ware that was not picked up by the other programs.

Good luck, and be careful before making an registry changes that are shown with hijackthis.