Yeah, use the 3.11 for playback as others say. If you want one of the best players, with THE best GUI and options, then get Sasami2k! Only faults I can find with it most people won't care about like; No spdif out, no onscreen playlist, and no 10band eq in the latest version.
There is 1 reason I can see for getting the playa. You can correct aspect ratios with it, as I find many encoders can't seem to get those right.
For WMP7.1 I like the way the playlist is done, but everything else on it sucks.
Forgot to mention wmp6.4 which still exists after 7.1 install. Just run mplayer2.exe to get a nice solid player without the frills.