ICE for Windows

No new program files means:* no new malware* no new spyware* no new ad-supported screen savers* no new toolbars* no redundant software* NOTHING

It also means that you won't be screwed just by opening an unsafe email attachment or clicking on a link to an infected website.

ICE can be paused for a short time for legitimate software to be installed, but most people never need to install anything else after the computer is set up.