p0sixspwn, the jailbreak tool for iOS devices running iOS 6.1.3 through iOS 6.1.5, has been updated for Windows users. Originally released exclusively for the Mac, p0sixspwn makes jailbreaking possible for those who wish to remain on older firmware, or for those who can't upgrade for whatever reason.
The jailbreak tool is the result of a collaboration between iH8sn0w, winocm, and SquiffyPwn. You can find the download links to the Windows version and the latest version for OS X inside.