We announced the greenpois0n jailbreak for iOS 4.2.1 over the weekend. At the time, the jailbreak tool only worked on Mac. It didn't take long for the Chronic Dev Team to update the tool to now support Windows. This means you can jailbreak your Verizon iPhone using either a Mac or Windows.
As always, make sure you know what you're getting into when you jailbreak your iPhone. You should definitely read the documentation included with greenpois0n before you alter the way your iPhone works. The instructions for executing the jailbreak are pretty straightforward, and you shouldn't have much trouble, even if you're new to the jailbreak world.