Please confirm whether this is happening with all the documents or is it related to a specific document.
If it's happening with all the douments then Navigate to C:\Documents and Settings\Current User\Application Data(This is a hidden folder)\Quark\QuarkXPress 6.0 folder and copy all the .prf files in some new folder on the desktop.
Now Relaunch QuarkXPress and check the results.
Regards
Karam