It could be in a number of different places in the registry, the first place to look is
HKEY_CLASSES_ROOT\.txt
(where .txt is the extension that is having the problem with extra \"Open with\" entries)
There might be a subkey named \"OpenWithList\" under this key, look in there and if the video player is listed, delete it.
Another subkey that might hold the info is the \"OpenWithProgIds\" key, again delete any bad entries.
IIRC there is somewhere else in the registry that also can hold this info, but those two locations are the primary suspects...