Quicktime For Mac and/or Windows

If you save your files in .mp4 containers (using any applicable codecs other than h.264), there should not be an issue. PC users will find WMP10, VLC, MPC and DivX player as alternatives. I'd suggest VLC above all others for maximum compatibility. There should be no hesitation in downloading VLC.

Otherwise, if everyone is moaning about the format, use ffmpegx to convert your .mov file to DivX AVI and keep the PC crowd off your back for good.

A QuickTime MPEG-4 export (.mp4) will require QuickTime Player for Windows to open the file.
Microsoft doesn't follow the industry "standards" and instead uses their own implementation and a proprietary video codec to make MPEG-4 videos.
Media Player on a PC can't open and play a QuickTime encoded MPEG-4 file.
The only QuickTime for Mac export that will open on a PC is the now out of date .AVI container.

"A QuickTime MPEG-4 export (.mp4) will require QuickTime Player for Windows to open the file"

I'm hardly an expert compared to QTK by any stretch, but I think I've posted elsewhere that I spend a good bit of time on a closed site where PC and Mac people exchange "real world" information about the ability to share video in various codec/container configurations. The intel I gain from that forum is invaluable for my line of work.

Quicktime installation on a PC is not necessary for Windows to open a .mp4 file produced on a Mac using QTPro. As a test, I made an 8MB clip from a HD trailer on Apple's website, cropped a scene and did a simple export to MPEG-4 Improved format, dumped it in a mp4 container and posted it for my PC folks to test.

The video plays with perfect picture/sound quality in WMP 10 (ffdshow), Nero Showtime 2.0.0.39, Real Player (Win32) v10.5, QTA 1.6.9, MPC 6.4.8.9 & 6.4.9.0, MPlayer 0.0.9.0, ZoomPlayer 4.5.1.0 and of course, VLC 0.8.5.0. There are bundled QT components available (7.0.4.80) for install that allow WMP and MPC to work perfectly on the PC with Mac-produced .mp4 video.

Eight PC video players will run the file just fine.

TAV,
I sit here and type with my foot in my mouth and sorta knew it when I hit the "Post Message" button.
There are QuickTime MPEG-4 exports that will "play" (and edit, etc.) on a Windows OS.
Microsoft has developed proprietary MPEG-4 "compliant" video codecs. They can be found at this link:
http://www.microsoft.com/windows/windowsmedia/player/faq/codec.mspx
And more about the "standard":
http://www.mpegif.org/
But, too many users feel that a "one codec fits all" approach and create files that are really "player" specific.
Most Microsoft MPEG-4 files will not play on a Mac OS via QuickTime without the Mac user installing third party video and audio codecs.
The same is true of Mac users QuickTime exports.
Exceptions are always the rule and your mileage may vary.
Just exporting to MPEG-4 codecs doesn't guarantee playback on both OS's.

No worries. I know very little vis-a-vis you about how this all works. I simply know that sharing cross-platform has become more and more necessary. In my line of work (movie trailer production) - it is simple enough to ship a D2 Master from point to point so we're all looking at the same thing. Or we simply video chat and run clips Mac Native using a video client. But when we need a quick fix for someone, honestly I simply take whatever comes out of the Edit Bay, crop and save as a .mov file in QTPro, and clients use VLC to play it. It might be an abortion (source XviD AVI slammed into a .mov container, for example).

Of course the ad/marketing geeks want it native so they can pull it apart and look frame-by-frame, but the higher-ups spending the $$ are all on PC - so VLC - oddly enough as a free player - is the most platform agnostic thing around....