Backup everything on an infected machine before you try to clean it. Simple mistakes or miscalculations can cause data loss.
Most viruses are pretty easy to remove.
The ones that are toughies are usually the most rarely seen. Read about the virus you're up against. Good sources of info include Patricia Hoffman's VSUM, and the virus info included inside of F-Prot. Forget to update your scanning software. New viruses are found on a daily basis.