Ah, Vista..... LOL
Lee - open youtube and start a video as normal, then right click the flash video and click 'settings.'
Then select the left tab and uncheck 'hardware acceleration' and retry (close and restart IE).
If Vista won't let you uncheck the hardware acceleration, try this:
Go to appearance and personalization in Vista and adjust screen resolution.
Reduce colour to 16 bit temporarily. You can now go and uncheck the box in
flashplayer at youtube etc. Then simply go back and reset the colour to 32
bit.
Oh, and make sure you're signed into Vista as user with Admin rights if you have multiple accounts on the PC.