The easiest probably is to install a Linux dual boot and get or compile and execute radiant there.
Or set up a development environment for UFO:AI (or rather, radiant) on windows and try to fix it yourself.
Of course, it might just happen that that binary that's provided for download works on win xp but not 7, or that something like "execute in compatibility mode for xp" does the trick, as the system that's used to build those executables very likely is xp.
It might also help to try to get some info out of the program about why it doesn't work, such as a backtrace (does it crash ?) or debug messages. In some programs those can be activated without needing to compile it.