Hi I have the same operating system (Windows 7) and the latest version of Windows Media Player (I think it's Version 11) on both my desktop and laptop, however while my laptop plays all DVDs fine my desktop doesn't. I click "play DVD" and it says "Title 1" in the corner then returns to the "Play DVD or Go to Library" menu. I click and it repeats the process. I have also tried Cyberlink PowerDVD as an alternative on my desktop but that freezes on a still image while the time recorded seems to carry on. Some DVDs the desktop will play fine, but others it simply won't.
Update: I just checked my old DVDs (sorry should have done that before) and the desktop is no longer playing them, so it's not playing any DVDs at the moment - same errors on running any DVD. It can read them in Explorer though, but very slowly.