IT Trouble Ticket Software?

I would like to set up a small help desk type system at my work, just for submitting and prioritizing/organize user tickets. Currently we have no such system, and if a user has an issue, they come back to one of us and stand at our desk until we go fix their problem, regardless of the importance.

I'd like to create some way for users to submit a ticket from their desk, where they wont disrupt what we are currently working on. The ticket will then be organized in some way.

The catch is this has to be Windows based and free (or very low cost). I've seen some help-desk software, but all of it seems to be Unix required.

Any help is appreciated.