The unfortunate truth is there is no simple answer to your problems. Site advisor and Defender will (attempt) protection against spy/mal-ware but not viruses. Without a tool to find/remove viruses you are going to have a tough time getting your computer clean. Manual cleaning is time comsuming and requires extensive knowledge of Windows (registry editing, etc.) and the viruses themselves so you know where and what to look for. Here are some things you can do:
Apply all current patches from Micorsoft.
Stop using Internet Explorer for web browsing. Get Firefox.
