In general with any raster graphics, whether its a photo image or a MPEG movie (which is just fast moving pictures) you can't get better then the original. There are little tricks but they mainly consist of filters blurring, smoothing, and duplicating pixels of the same information. Its not really increasing the true quality.
To get a truely better quality movie, you need a better original.
Anyways to the original poster, rimmer, I was assuming you meant MPEG's and AVI's you have already. Now if you are recording or converting uncompressed video to MPEG or AVI, then thats a whole differnt ball game. You can recode that to have different levels of quality. Still, you can never get better then the original. If anyone wants a full explaination I will give one, but for now, think of it like scanning a photo. If I take some junky shot with a polaroid, you can scan it all you want at 2400dpi but it will never look any better. You can print it on a $50,000 die sub printer and it still won't be any more detailed then the original Polaroid.