Here is a good Jailbreak guide for this.
You will need to install the new version of Redsnow and the current IOS 4.3.1 from Apple for your device.
To install 4.3.1, simply hold the shift key, and select Update or Restore, depending on what you want to do. Then browse to the 4.3.1 file you downloaded.
Make sure you run redsn0w as administrator, otherwise it WILL hang when updating the Ramdisk.
If you did an upgrade, you will also need to re-install your Cydia specific aps, such as Installous, Winterboard, SBSettings, Pandora skip hack, etc. Those aps installed by Installous will be there and fine.
The above is for Windows - OSX can suck a fat one.