After releasing Redsn0w 0.9.8b1 for Mac, iPhone Dev Team now released Redsn0w 0.9.8b1 for Windows to tethered Jailbreak iOS 5 Beta. The Jailbreak is only for those having Apple developers account and can access to iOS 5 Beta version.
Redsn0w 0.9.8b1 for Windows can jailbreak iOS 5 Beta on iPhone 4, 3GS, iPod Touch 4G, 3G & iPad 1.
Since Apple now provides Windows iTunes 10.5 for iOS5 and iCloud developers, it’s no longer the case that only Mac owners can legitimately activate their devices. We now provide a Windows version of redsn0w for those developers (only!).
Unlockers especially should stay away from this because restoring to iOS 5.0 will upgrade your baseband (unless you use Sn0wBreeze) and UltraSn0w has not been updated for 5.0 yet.
The step by step guide to jailbreak iOS 5 for iPhone, iPad and iPod Touch is posted here which is same for both Mac and Windows users.