If you haven't updated to iTunes 8.2, DFU restore to a clean 2.2.1 (non-Jailbroken), if you have updated to iTunes 8.2, do a normal restore to 2.2.1.
Then, in iTunes, click the update button. iTunes will download and install 3.0 on your iPhone.
Voila, you're now on iPhone OS 3.0.
You will not stay Jailbroken. You should never update from a Jailbreak. You should always restore then update first.
If you want to Jailbreak 3.0, wait for the Dev Team to release their new 3.0 tools (PwnageTool and QuickPwn. You'll be using QuickPwn since you're on Windows). Once you've got the tools, follow the usual Jailbreaking process and you'll be done. Congratulations, you now have 3.0 Jailbroken.