Three other tricks:
For Windows XP, 2000, etc:
Click on the DirectRT / MediaLab folder (the entire folder) and instead of read / write, select 'full control'.
Also, if the code still doesn't remain stable after making these changes, delete the file 'license.dll' from the MediaLab / DirectRT folder. Restart the program, then see if the new number generated remains the same.
I have known of one user who found that made a difference.
For Windows Vista and the Beta version of DirectRT:
When logged in as an admin, try deleting the file license2008.dll and then start the program again. The file will be re-generated. See if that gives you a stable code when you open and close the window a few times.
Once the code is stable, we can send you an activation code which works.