You'll never be able to stream Full HD files wirelessly, there's just not enough bandwidth available, 720p files sometimes work, just depends on the size of it.
TVersity or WMP11 shouldn't make much of a difference to be honest. As already said, you'll need a beast of a pc to be able to transcode HD files on the fly and play without stuttering. The only thing i've not been able to play with WMP are .vob files, but you can just rename them to .mpg and they'll play fine.
If they are "normal" i.e. non-HD files you're trying to play, then there should be no problem wirelessly playing them, unless of course your signal strength is very low, so as has been said, it's worth giving a few more details as to your setup....
TVersity was good for me, until i switched to wired, then for some reason it didn't work at all!