"Unknown error" (or "0x80040154: Class not registered"). P..

Archived from groups: microsoft.public.windowsxp.help_and_support (More info?)

I get "Unknown error" message when I'm trying to play a file (any file) with
Windows Media Player. I use Windows XP SP2 now. Some other players experience
the same problem while trying to play f. ex. .avi-files. In addition, Roxio
CD Creator denies to burn CDs (error: 0x80040154 "Class not registered" ).
I have experienced some of similar problems before. That time it was
something with .dll's, so I had to run regsvr32 ...dll.
What should I do to repair it to be able to play media files and write CDs?
I'm almost sure that it's not a codec problem or smth like that.
Thanks!