As joonas stated, there are some new cvars related to mouse behavior. The m_xp cvar fixed mouse acceleration for windows XP, but when Windows 7 came out, it was a different version, so it still enabled mouse accel. You can reenable it, but you're a lot better off upping your sensitivity and learning to play without accel, as it will be much more consistent (mouse movement always turns the same amount, regardless of speed). Also, the FOV scales your mouse speed automatically, so if you zoom in, you don't have to have elaborate aliases to adjust your sensitivity. If you actually take the time to adjust your settings, you should be a lot better off.
MosEz: I have a similar issue on my laptop, but I've had it for as long as I can remember. It seems when a new sound plays, it hitches (I guess it has to load it from disk). The sounds should be precached, so I'm not sure why this is happening. It's not a new thing, though. I don't think the sound loading code has been touched in years.