Looks like I spoke too soon. The problem still exists and judging from the
resource manager it has no correlation with the disk activity but of course I
could be wrong there.

And to be clear about the setup I use, I have Vista on the hdd controlled by
intel ich5 and the promise 378 only controls the raid unit which only holds
my music, videos and games. But the problem still occurs when playing video
from the other hdd as well as when the computer's just idling.