Do you mean to say you replaced your CD drive with a DVD drive? Do you see the DVD drive in Windows and can't play DVDs? Or does Windows not see the new drive at all?
If you can't seem to play any DVDs, download VLC media player, it's free and will allow you to watch DVD movies if you don't install any of the software your drive may have come with. Most DVD drives come with WinDVD or a similar software.