Redsn0w -- the lightweight, multi-platform jailbreaking, unlocking and customizing tool for the iPhone 2G, iPhone 3G, iPhone 3GS, iPod touch 1G, iPod touch 2G, and iPod touch 3G -- has been updated to 0.9.6 Beta2, which allows Custom IPSW installation with Windows. Missing from this update is iPad and AppleTV support, which is currently supported by pwnagetool for OSX, but word is that chpwn is working on a windows version of pwnagetool.
Redsn0w can be used to install custom IPSWs that will keep your baseband firmware from updating and allowing you to unlock with ultrasn0w. Redsn0w works by putting your device into a special DFU mode which is required for installation of custom IPSWs.
The iPhone Dev-Team has also posted an update on redsn0w+limera1n fun: