Thanks a lot wahnula. I was able to run the defrag program from safe mode. But when I go back to normal mode I can't access the program any more.
Also on the errow message is "This may be because the file does not exist, is not an MMC console, or was created by a later version of MMC. This may also be because you do not have access rights to the file." I seem to think it is the latter reason, but I was always able to access the file before. I am the only one that uses this computer as administrator, but there is also a guest account that noone uses. Is there a way I can permit all users to have access to all programs? Thanks a lot for your help Tony. Bob