RTC is locked to our on site GPS so I don't think it's that. I also disabled the corporate firewall (which disables the Windows one on installation) but that makes no difference. I've found one machine that it works on but sadly that's my dev machine which has all sorts of crap on it, too much to go through one by one to figure out what it is.

Another option would be to rewrite the app and make client/server and just run the server element on one of my linux boxes. This is actually likely to be quicker.