The last beta version of Redsn0w 0.9.7 (i.e Redsn0w 0.9.7b4) eliminated the need of seperate Usbmux process making it easier for OS X users to untethered jailbreak iOS 4.2.1 on their iPhone 4 & iPod touch 4G. However it still requires iOS 4.2 beta 3 firmware files & saved iOS 4.2 b3 hashes.
But if you are Windows user and fulfill all requirements of Redsn0w 0.9.7 then here is good news for you the beta version i.e Redsn0w 0.9.7 beta 5 will come with Windows support. Here is what Musclenerd said in reply to http://twitter.com/nyzilleztndn/status/21341610151247873 tweet.
So get ready with your 4.2 beta 3 firmware & iPhone 4/iPod Touch 4G for untethered jailbreak iOS 4.2.1 on Windows.