Chronic Dev has finally released their jailbreaking tool Greenpois0n
for windows users. They were off to a bumpy start this morning, and the current(as of 8 am EST) is RC3. Just like other boot level jailbreaking tools, this requires you to place your device into DFU mode. Run Greenpois0n, and it will take you through the required steps.
After the jailbreak is done, you will have a new application on your springboard called Loader. Use it to install Cydia, then it will ask you if you want to remove it. You can, or you can keep it for nostalgia. A quick re-spring, and you now have Cydia. You know what to do next.
Mac users will have to wait for either a separate release, or load up Windows in Bootcamp. Virtual machines are not supported, though we have heard reports of Parallels VM working, but not VMWare.