Most professional audio post houses use Pro Tools. At the indie level some are using Logic (Mac). On the PC Nuendo is used and the audio implementation in Sony Vegas is pretty good; it started out as a DAW.
Filmmaking is the art of the invisible;
If anyone notices your work you haven't done your job right.
If you're talking post-production DAW, it's Pro Tools, hands down. There's a good following for Nuendo, but all sessions, stems and elements shared between facilities are mostly PT-based. With PT9 opening up formerly TDM-only features to native systems, I think PT penetration in the lower-budget productions will continue. Logic is hard to beat for composing music to picture, but post-prod is clearly not a priority in its development. Anyways, you said "PC", so let's forget Logic.
So it's Pro Tools. Or Nuendo, if you're feeling rebellious. Or Pyramix, if you're French.