This is the best and easiest to follow method for jailbreaking the iPhone 3g on Windows.
Disclaimer: Jailbreaking your iPhone could void your waranty.
I am in no way responsible to any damage that may result by following this tutorial, so proceed at your own risk!
This method works really well and does not require any patches to get iTunes working.
Before you start, make sure you are running iTunes 8 and make sure your iPhone is running version 2.1
To see what version your iPhone is running goto Settings / General / About on your iPhone.