Have you tried "systems restore", back to before your download?
Beyond that I have no ideas, because I'm fairly new to all this myself, but what I can say is that you will need to provide more details, e.g. are you running XP and what version, before anyone can help much.
So I put in a new Dvd drive in my computer recently and it has given me no problems until now. I downloaded a dvd decoder for Windows Media Player, and once it was installed tried it out. The DVD played but I tried to skip ahead on the DVD and media player closed on me. And it kept closing on me every time I would try to play the DVD.
I restarted the computer to see if that would help, and when Windows started the only thing that came up was the desktop background. All my icons aren't there and the taskbar is gone.
Can anyone help me?