Very well could be a problem with the files you're trying to play, but the first thing to do, IMO, would be to uninstall Windows Media Player, reboot, reinstall it, update it, reboot, and try it again. Another option to try might be an alternate media player - but be careful, lots of the free ones come with unwanted tag-along freinds.
IrfanView is a fairly usefull all-around graphics app - and yuckware-free (as long as you don't install the e-Bay toolbar). There are some others listed
HERE, but, as mentioned, be careful; check 'em out, or ask here, before downloading and installing something that's gonna wind up being a sure road to bigger headaches.