Ulyoath, i also have some fps drops with horses. Not in the arena, but in larger battles. Especially when they're traveling through water(30fps or so). I've also noticed that units on the other side of an obstruction (like a mountain) still hurt your fps. Perhaps a few changes to they way units are filtered could alleviate this problem. Here's my specs.
Amd athalon 3000+
512mb RAM
radeon 9800pro
So, it really shouldn't be lagging me like it is. I mean, when you're getting around 200fps, and all of a sudden it drops to 60 when you look at your units, its noticeable. And no, i don't have any viruses/spyware, just formatted less than a month ago.