Windows Steady State is a great app -- but it's not going to be sticking around much longer and it doesn't work with 64-bit Windows. Returnil System Safe is a nice alternative which does support Windows x64, and while it's normally $40, you can score a one-year license free of charge right now.
Returnil offers virtualization-based protection of your system, as well as built-in malware protection. It's also got a handy system rollback feature that lets you recover individual files that were changed, or set the entire system back to the way it was at your chosen point in the past.
With virtual mode enabled, a quick reboot is all it takes to completely undo intrusions from pesky malicious software. It's a great way to keep your less-savvy friends from getting themselves in trouble online. Even if they do install Super Antivirus Pro 2011 Gold Plus and have bogus alerts popping up all over their screen, all they've got to do is shut down and start again to make it all go away.
Registration is required, so use a disposable email address if you've got qualms about giving out your primary one.
Download Returnil System Safe Pro