Try Playing them in Media Player Classic (Comes with the K-Lite Codec Pack)? Or, VLC
If anything will play them, VLC will.
Can you still play the ones that worked before? Or, did it quit playing period?
K-Lite Includes Divx, So, that shouldn't have been an issue.
You can also try running the "Codec Tweak Tool" That came with K-Lite. It's supposed to repair broken Codecs.
Start >> Programs >> K-Lite Codec Pack >> Configuration >> Codec Tweak Tool.
Then, Check The "Fixes" option at the Very top, and the "Reset to Recommended Settings" option, and hit next and see if that does anything.