Have you compared this with scrolling in any other program? In Firefox I get higher CPU% usage than mIRC.
GDI in Vista is a very intensive process because unlike XP, it's not hardware accelerated, meaning updating the UI is the CPU's job; it's not surprising that a lot of scrolling will generate high usage. Even with hardware acceleration, you should expect that the OS will devote prioritized CPU time to active UI tasks. The question should not be "why is my CPU at 100%" if there are no other programs that need CPU at the same time, that should actually be relatively normal. Also, for how long is this % being used for? If it's just for the duration of the scroll then it's pretty normal as well.
