Recently I wanted to put some of the diskimages of one of the Millenium CDs on harddisk. When I tried to run them on an emulator I sometimes encountered errors like garbled graphics, error messages and stuff like that. Could it be the diskimages on the cds weren't all error free to start with or are my CD starting to degrade in some way?

There are indeed some bad disk images on these CD's. For many of these bad images, I've succeed to find on Internet better files, but some luck is required.

There are also disk images that are very difficult to do correctly, especially when the disk is anticopy protected : for example, Fac Demo 5 (but you will find here on MRC a fully working deprotected version) or Sunrise Diskmagazine 2 (no-one has succed to make a fully working diskimage ... I've tried it myself, as I have the original disk, but I have only found a part of the solution until now).

Unfortunately there are also quite some errors on the CD. But the biggest part should be just fine.