Nope. There is slight "aim acceleration" only on the consoles.
Edit: Note that acceleration isn't the same thing as assist. From H0RSE's compendium:
“CoD, and most console FPS games actually do have a short amount of aim acceleration to them:
The first couple of frames are "catching up" to your input speed, so that "flicks" or "taps" in direction result in small angular steps, and not in full speed jumps, which tend to just lurch your aim over things you're trying to finely aim at. In short, it's pretty frustrating to have no acceleration what so ever, but we keep ours to a minimum, like CoD.
We don't, however, do CoD's single-player only style of aim assist (momentarily snapping to a target on the "positive edge" of an ironsight button press).”
- Aubrey Hesselgren, Lead Technical Designer