Mac users have little to be concerned about re: viruses. Except if you want to play wma files, I'd recommend VLC - (click on link) _ (choose nearest mirror site) so no WMA files can force a popup window or redirect you to a dialler site. Using other players will do this. VLC won't allow it to happen!

You didn't say which mac os you're using but there are no OSX viruses at present. But still, if you're sharing such files, make sure you scan them with a AV program 1st so it doesn't potentially affect other windows users should the file be suspicious.

I personally always try to stay clear of wma files. BTW there is a couple of wma converters out there for mac; ie: convert to aiff or mp3. The previous the better.