Jimbo is correct in that VLC SHOULD pretty much solve your media player problems.. I'd also sugest getting that, it works extremely well and is Free...

If youtube is the only thing holding you back from getting VLC than why not try a different browser... if your using IE 8 right now, than maybe try getting Firefox and see if it works any better for Youtube... If your already using Firefox than try Youtube in IE 8 if you haven't already... If you've already tried both browsers and neither is working out good for you.. than maybe it's a driver problem with your Video Card...

Which gpu do you have and which driver did you install for it?....