[id10t]

Format, reinstall, *dont* use IE for anything but windowsupdate.microsoft.com, use a hardware router/firewall, have all service packs and patches applied before even connecting to a network that has another windows machine on it, use good antivirus (mcafee or norton) and keep it updated weekly (or every boot), be sure to run the automatic windows update daemon, try to avoid using P2P but if you must use a open source client (that has ports to other platforms as well as windows, like mac and linux), and *don't let anyone you aren't married to use the darn thing - computers are cheap, get their own*. That last part includes kids, especially teenagers.

Or, if you aren't tied to a particular windows-only application get a Mac or install Linux (or just use a live cd like ubutnu or knoppix).