Networks VM/VA proxy to change user-agent

Alright, I need your help guys. I am after a proxy that I can run on a Windows box (preferably) that can change the user-agent of a request. There is currently a proxy that only allows IE or Firefox user agent requests through and we need to use a proxy before this point.

Ideally it should be a small VM/virtual appliance we can download but anything that achieves the desired result will do.

Thanks.