1) Does this happen in Firefox or just IE7? And what about Opera [which has beautiful scrolling]
2) What is your videocard? Latest forceware?
3) Have you performed any CPU Quantum tweaks? If so, set it back to Server Defaults and try again [set Program Prioritization > Background Services for easy way]
4) Have you tried the 'Low Priority Reservation' tweak to set it to Workstation rather than Vista?
5) Does it happen when you scroll only with mousewheel? Or also with keyboard up/down arrow keys? Are your Mouse and Keyboard both USB, both PS/2, or one and the other?
...times like this i'm glad I took a chance on buying an ATi that was cheap