Troubles
Websites
WMP mini FAQ by zachd contains a lots of trouble shooting tips.
Search the Microsoft Windows Media Player forum.
DirectX
Run DxDiag if you have problems with DirectX
Double entries in the library
If you open an album, all songs are duplicated.
This can happen if you decided to move songs to another directory.
A solution can be found here.
Edits lost.
Complains like WMP keeps on changing my tags, WMP is reversing all my updates, are due to using the online database (find album info) and the automatic update option.
If WMP finds an album in the online database, it writes all the info in the library and in the tags. You edit these tags but as soon as an automatic update is applied, your edits are overwritten when the overwrite option is in effect to.
How to control this can be found here.
Media sharing not working
Media sharing is not file sharing. It is streaming AV using UPnP
A bit more about trouble shooting streaming AV.
Drop outs
Play back might be interrupted now and then by other programs.
Anti-virus programs polling the internet at high priority for updates are a known source of problems.
Put the task manager or the Resource Monitor on to locate the process.
Look for CPU peaks, heavy disk access or network traffic.
Setting the player priority to high might help too.
More about trouble shooting drop outs.
WMP won't work at all
WMP is an integral part of Vista, you can't un/re/install it.
If it doesn't work at all you might try running "sfc /scannow" from "cmd.exe".
You must be Administrator to do so.
System File Checker gives an administrator the ability to scan all protected files to verify their versions. If System File Checker discovers that a protected file has been overwritten, it retrieves the correct version of the file from the cache folder (%Systemroot%\System32\Dllcache) or the Windows installation source files, and then replaces the incorrect file. System File Checker also checks and repopulates the cache folder.
Source: http://support.microsoft.com/kb/310747/