what is ClassFactory?

I have just recovered pis from a CD and there is also a
few AVI files. When I click to play these on media player
I get the message "ClassFactory cannot supply requested
class" These AVI files were created on the same PC (prior
to a system restore) and were working fine. I have
checked the properties on each file and it all adds up
(around 1 - 2 MB/ file, so I assume everything is there.
Can anyone help me as to what the problem is? Thanks in
advance, Crosby