I don't really see why you would have to use a different OS. OK, Ubuntu (Linux in other words) could achieve to do the thing you want to do, but in Windows (not that I am such a Microsoft fan, but the possibilities are there) you have (or can have if you want) all the software you need to achieve using your PC as a CD/DVD player.
You might have to do a bit of tweaking to your Startup folder (at least, that is what I seem to understand from your post, namely, that you want to boot up your PC and immediately put it to use as a CD/DVD player), but there is Windows Media Player for playing media files (including CDs and DVDs), there is a Windows XP Media Center Edition, which has a complete Media Centre to play media stuff (a bit more extensive than Windows Media Player), there are lots of excellent (and some even free) media programs, for example VLC Media Player, which can be found at
this location. Once again, you would have to tweak the start-up order of your PC, but it would all be perfectly possible. So, there are a number of possibilities you could explore.