Ok, no one seems to help. If you want to talk about Mac why don't you post in the Mac thread?
What I would do my friend is this:
Type in msconfig.exe in your run command and then when it loads up go to the Startup tab; once on here uncheck everything. Restart your computer and then see if you're able to install an anti-virus program. If you are, then get the anti-virus program to do a full system check at the booting stage.
However, there is a good chance you will not be able to open msnconfig.exe because quite a lot of virus disable these sort of things. In that case you can either try to fight the virus battle by getting anti-virus programs to try and work on your PC, with their fancy boot discs or what not. Or, you can do what I would do and just install XP again with a nice format.