1) after download Proxomitron, extract and run proxomitron.exe(the green icon).. If any pop-up come out from your firewall just allow it..
2) In Proxomitron main window, "Edit Filter" section, click "Headers", then a "HTTP Header Filters" window will come out..
3) In "HTTP Header Filters" window, scroll down and you will find there is 6 user-agent option.. Choose  any one of them by ticking at "OUT" section and then click "Apply", "Ok"..
4) Configure your BT client to use this proxy..
5) Launch Proxomitron log windows and then start your torrent task to see if it works..

Proxomitron was using port 8080 on default so if u're behind a router, forward port 8080..