Video decoder error

I have an Inspiron E1505. OS is Win XP MCE. When I try to play a DVD in Media Center and Windows Media Player, I get an error indicating that the there is a video decoder error. In MCE it tells me to restart MCE and/or computer, but that doesn't help.  How do I reinstall whatever video coder is now missing from the disks provided when I bought the computer? Or can I download something from the Dell site?