Suddenly ArcCatalog won't open...?
The ESRI user forums saved the day once again. I couldn't get ArcCatalog to work all of a sudden - it was taking a very long time to open, and when it finally did open, it wasn't responsive and kept hanging. If you are experiencing the same problem, try renaming your Normal.gxt and ArcCatalog.gx and let ArcGIS recreate them on its next startup. The problem could be that they have become corrupt. You will find them in the folder C:\Documents and Settings\User Name\Application Data\ESRI\ArcCatalog - where User Name is the user name you use to log on to the computer.
I renamed both files, and ArcCatalog started up in no time.