Not had this problem before so has me a bit stymied. I've tried to play clips on BBC News but all I get is a green viewing screen, the sound is fine. I downloaded a demo video of a game and it has the same problem. I've tried using both WMV and Realplayer as well as Realalt. I can put a DVD movie in and it plays fine using Realalt. I'm running WinXP(sp2) DX9.0c with a GeForce 6600GT Graphics. I use IE7 and F'Fox. As far as I can tell I'm up-to-date with Win updates and have the latest NVidia driver and my mobo drivers are current.
System restore isn't an option I'm afraid and 'Jetaudio' you pointed me at appears to be concerned with audio which I don't have a problem with. Thankyou for your input skidzy I appreciate it so if you think of anything else It'll be most welcome.