Hyperthreading causing system slowdown during keypresses.
I normally don't use HT, although I did briefly when I first got my new CPU [P4 2.83ghz, and it worked fine], but I decided to use it again. When I enabled it, and rebooted a few times, it was enabled perfectly fine, with a byproduct being that whenever a key is depressed my entire system slows down for the duration of the keypress. Music, cursor, processes, everything. It also seems that when a key is depressed my CPU load jumps to somewhere around 50%.
I've been using/building/configuring computers long enough to know that I can't be the only person with this problem.