Here's another solution/workaround from another BB forum...
The typical path on Windows to the "Shared" folder is:
C:\Program Files\Common Files\Research In Motion\Shared
Originally Posted by vaca232
Just rename the "Shared" folder inside the "Research In Motion" folder, or the "Loader Files" folder. This prevents the DM from realizing you have an OS installed on the computer, so there isn't an option for "Blackberry 4.7 Core Applications" anymore.
When you need to install an OS, just rename the folder back to its original name. I just stuck a bunch of ZZ's at the end of the name so it's obvious.