9550 mpeg problem

I have a hightech radeon 9550, when I look at mpeg files (eg dvds, streamed web content) I see a wierd image - lots of primary colours, can't really see image. If I turn off hardware decoding in the dvd player software the image is ok (tho myt pc lacks grunt for this really). I had this happen before and manage to fix it (i think by changing a seemingly unrelated driver) but can't for the life of me remember what I did. I have tried all sorts of different versions of the ATI drivers to no avail. Any suggestions?