Hello Algatelli, and welcome to Seven Forums.
Was this in
Default Programs that you tried to set
Windows Media Player as the default program with all of it's associations set to it as well?
You might also see of uninstalling RealPlayer SP Basic may help if you have not already. If you did, and the above does not help either, then you can do a
system restore using a restore point dated before you installed this program to restore this back.
Hope this helps,
Shawn