You know that on the Mac OS, you can put comments on files by clicking on a file, select Get Info on the File menu and type in the comments. Well, I found that all the comments are store on one of the two invinsible desktop files "Desktop DB" and "Desktop DF" in the root directory of the hard disk.
The comments will be kept if you rebuild the desktop by holding down the Command and Option keys at startup. But if you create a folder on the desktop for example, use a utility like File Buddy and move those two files into it, restart the computer and the computer will create two brand new desktop files. If you delete the folder on the desktop containing the old desktop files then all the comments will be lost.
I could imagine that if you format the hard disk, all the comments will be lost also, so be careful with your comments!