Have you installed any codec pack? If the answer is YES then uninstall it (click on Windows START button -> Control panel -> Add / Remove programs -> Search for a codec pack or similar name)
I ask you about codec pack since you said you don't know where they are installed etc... Once you have removed it (if you had installed it previously) use
GSpot to know which codecs you need to play your movies, then go to
this adicoto's post and download the codecs you need (probably you'll only need to download ffdshow since it can decode almost all video types)
OFF TOPIC:
WOW GSpot has been updated in these months and I have not updated it yet :shock: :lol: