This may sound silly, but check that your not in "close up" mode or something like that.
Its natural that the video would look better on your phone than a pc, but if the degradation is high, then it may be a fault.
Check out the rollercoaster ride video posted on the main site today for how it should look on your pc.
Are you uploading them from the phone to skydrive? check your quality settings in your upload and see what they are set to.
Should add another thing. check to see if your camera lens is clean. It might have some oil etc. on it, on a small screen it might not show up, in a large screen it becomes visible.
There is a protective film over the lens and flash that is separate from the protective film on the back of the 920. I know it sounds obvious, and I'm not being mean, but I had a close acquaintance that made that mistake.
