The Real Player web site says:
Question:
Is RealPlayer supported on the Windows Vista operating system?
Answer:
The latest release of Realplayer 10.5 is compatible with Vista. The latest release is build 6.0.12.1578.
To find out which build of Realplayer you have:
1) Open Realplayer.
2) Open the Help menu, and select About Realplayer. The build number will be written in here.
If you have an out-of-date version installed, you can open the Tools menu and select Check For Updates....
(Took me about 15 second to find that information)