You didn't say what codec you're using in the .mov file.
See this FAQ entry:
If these are clients for whom you are making animations, then how are you delivering your animations? How are they being used? I would assume that a client taking delivery of animations would have their own ideas (even specifications) for such things.
Every time I send them a high res .mov file, they say it won't play for them
You are sending them files with a CoDec that doesn't exist on PC. You need to read up on what Quicktime CoDecs actualyl are available on PC. Aside from that, it's rather pointless still. If those clients don't even know how to deal with this or cannot install additional software due to administrative restrictions, that won't do you any good and neither does their request for an "AVI" - AVIs are just as much containers as are MOV fiels and could still have an unsuitable CoDec inside. So the proiper thing to do is to send them native H.264 files exported from AE using Adobe's encoder or WMV files. For the latter you are going to need the Flip4Mac extension...
As Mylenium says, for reliable Windows compatibility, purchase one of the Flip4Mac extensions.
For free, you can use MPEG4 as a fairly cross compatible option, but the quality isn't awesome.
Either way, you'll get best results using another tool to do your encoding, rather than After Effects.
Any reason why they can't accept image sequences?
Honestly, there's no real reason for Windows users to have any problems with Quicktime. It's usually because QT on their system is horribly out of date. I send ProRes Quicktimes to Windows clients all the time without any problem.
Have them download the latest version and see what happens.