Slashdot is reporting on a study from SANS Institute's Internet Storm Center which shows that an unpatched Windows XP box will be infected after about five minutes on the Internet. German researchers found that it took slightly longer, however the conclusion still holds:

"While the survival time varies quite a bit across methods used, pretty much all agree that placing an unpatched Windows computer directly onto the Internet in the hope that it downloads the patches faster than it gets exploited are odds that you wouldn't bet on in Vegas."