a relative was round this afternoon and was messing around on my pc and since then I have lost the ability to playback vidoeo files of any type through media player... Media centre, quicktime, real player and media player classic all work fine... just not media player. I have redownloaded the codecs and still no joy? any help would be appreeciated as this will annoy the hell out of me!

Try this:

Control Panel | Default Programs | Set your default programs.

Choose your player and set it as your default to open up all the extensions it can.

The second option can be used also if you need to customize things a bit.