There are two ways to find this information:
When you have reached the maximum number of installed computers for your license and you are attempting to install MSP on a new machine, the program will alert you of the fact that you have reached the maximum number of installations and present you with a screen listing all of your currently-licensed computers. At this time, you can choose to de-register any old installations where the machine may have crashed or is no longer in use.
You can also view your installations by going to MSP's License Login page and log in using your Activation code and password. At the top, you will see links that say "license information | billing settings | manage installations | SMS account." Click on "Manage Installations" and you will see a list of all your licensed computers. Click the "Deactivate" button next to the computer you would like to remove.