It's also happened to me. I think it happens when the existing session hasn't closed down properly (i.e. Realplayer is hanging in the background)
If what Serena has said doesn't help, try pressing CTRL, ALT and DELETE to get your task manager sessions. Go to processes and find your realplayer session (it will be realplayer.exe or something similar) and end the task. Then try it again. If that doesn't work or if you don't see the realplayer process name, restart your computer.