If you’re handing them full res files they should look ragged around the edges on motion. It’s called interlacing and computers don’t do it. Apple, being a more advanced machine, probably is doing some sort of deinterlacing or interpolation, whilst the windows macine is simply following orders.
I assume that they’re doing the web compression. So why wouldn’t you hand them the best quality files? Simply export qt movie and choose “current settings,” “audio and video,” and check to not recompress but do make the movie self contained.
Translated it means copy this video. If they have quicktime they should be able to use the clips on their machine and compress them however they want.