video decoder not installed or malfunctioning

I have a Dimension E521.  The Hard Drive died so I replaced it and reinstalled all the software that came with it I thought.  I used all the cd's that came with it, anyway.  When trying to set up media center I get the message. "The vidoe decoder is not installed or is malfuntioning.  Restart windows"  Well I have restated windows and downloaded any updates that Dell recomends.  If my computer came with a vidoe decoder,  It should have I paid extra for the DVD Reader Writer drive.  Why didn't the put the software disc in with it?  And Why can't I find the software anywhere on the Dell web site?  I have spend months of my spare time trying to fix this issue.

I feel like I would have been better off to have purchased a cheap computer from wal-mart and purchase all the software seperately.  Then at least I would have the software now to reinstall.  I have personally purchased 3 Dell computers and been responsable for helping 2 others purchase Dell computers.  I am not sure I can recomend Dell anymore.