There are many possibilities. The first thing to do is make sure that the program is still there. Look in \Program Files\Plant Tycoon on your hard drive and see if Plant Tycoon.exe is there. If not, it's likely that your anti-virus program has falsely identified a threat and removed it. That has happened quite a bit lately as anti-virus companies have changed the virus signatures they use for their scans.

If the .exe file is still there, then (if you're running Windows Vista), it could be due to one of the security features in Vista. There are several security features in Vista that can interfere with a running application. The most likely cause of this problem is that Vista's Data Execution Prevention (DEP) is interfering with the digital rights management (DRM) software that handles the activation code that unlocks the game's full features. DEP is an important security feature of Vista and should not be completely disabled; however you can disable DEP for individual applications when you encounter problems with the way Vista handles such applications.

To read about what DEP does and how to change its settings, click Start on your Vista taskbar, then click Help and Support. In the "Search Help" field at the top of the help window, type "dep" and press the Enter key on your keyboard. You should see several entries related to DEP, which you can browse to become familiar with this Vista feature. To change DEP settings, click on "Change Data Execution Prevention settings." Depending upon how Vista is set up on your computer, you may see that DEP is turned on for all programs. If this is the case, you can choose to either set DEP only for essential Windows programs and services, or to disable DEP for the game only by follow the instructions for turning off DEP for an individual program.