If you are going to edit the registry you do so at your own risk.
Back up the keys you are going to delete.
I have successfully remove visualisations from the windows media player visualisations list on windows xp using the following instructions.
Go to
HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/MediaPlayer/Objects/Effects
Here you will find keys relating to the visualisations, click on the key of the visualisation you want to remove then on the toolbar select File and then Export, give the file a name that identifies the visualisation and save it to a backup folder
Now open the key of the visualisation you want to remove and then open the properties key. Make a note of the classid number, the format looks something like this example
{0A767E66-4D5E-11d5-A8AF-00105AC582C2}
Select the key of the visualisation you want to remove again and press delete.
Now go to
HKEY_CLASSES_ROOT/CLSID
and look for the key with the number you copied down
Select this key, on the toolbar select File and then Export.
Give the file a name that identifies the visualisation and save it to the backup folder
In this key are 2 subkeys, InprocServer32 and TypeLib, open the InprocServer32 key and check that the default data points to C:\Program Files\Windows Media Player\Visualizations\name Of Visualisation.dll
Re-select the key with the long number and press delete Now when you open windows media player the visualisation has gone from its list