Of course, if you don't fancy messing about with Windows, most major Linux distibutions have had AMD64 versions for months now. With these, most packages are recompiled to take advantage of the architecture, so you might find it runs better, too
If you've got a spare HD lying around and fancy trying one, then Fedora (
http://fedora.redhat.com) and Ubuntu (
http://www.ubuntu.com) are both excellent free disributions with AMD64 versions.