I downloaded the free adition of real player 10 which seemed ok as soon as I opened it, then it  sayes real player has a problem and closes.  sayes "Docume 1\owner\locals\temp\b11d appcompat.txt " I have tried re downloading help First use www.ccleaner.com to download CCleaner and run it to clean your system. I am sure it will help.
If not check this link:
http://ask-leo.com/how_do_i_resolve_my_problem_with_appcompattxt.html
But I say you Real player is a headache. I use it rarely when very urgent online straem is needed. But as a default player it is really boring, if you haven't tried yet.Google appcompat.txt.  You'll get some findings.  I believe you need to OPEN that .txt file and see what it says.  I'm under the impression it may contain a clue as to what the problem is.  SINCE that file is in the temp folder, it's APPARENTLY not a regular program file.  I think I would try changing it's name, then opening Media Player and see whether that makes any difference.  If you no longer get the message and everything seems to work OK, then DELETE the file.  Actually, running Disk Cleanup should clean out the temp folder.

Do you maintain up-to-date ANTIVIRUS protection and do you scan your system for spyware?  Do you did able your antivirus software when you installed WMP 10?  If not, perhaps you should try reinstalling with your antivirus software disabled.Realplayer is a virus, Google for "real alternative".