Can you get us a copy of the error, I'm presuming you're seeing this error in Control Panel - Services, or in the NT Event Viewer.
Are you sure you don't have some other application on that server that is listening on the Radius port(s)? Open a command prompt and do "netstat -a" which will show you the open ports, search for UDP 1645/1646/1812/1813, if these are open and the CSRadius service is not running, then there's something else on that machine that's doing Radius and that's why the service isn't starting.
