The 30GB is probably how much room you have on your harddrive. The 512MB memory requirement is not referring to space on your harddrive, but how much actual memory (RAM) your PC has. You can view how much RAM you have by going to Start->Control Panel->System.
The minimum RAM requirement for the game is 512MB, which is what the message box you are getting was indicating. The retail disc installer will prevent installation if you do not meet this requirement. Having a lower amount of RAM will make the game run slower. It is still possible to run the game though. Since you are being blocked by the installer from the retail disc, let's bypass it altogether...
In order to keep your game up to date once it is installed, you'll use our free application called Impulse. You can register and install games using Impulse as well, never needing your retail disc.
So here's what you can do:
-Go to the Impulse website (http://impulsedriven.com/about.aspx) and download Impulse.
-Find your registration key from your retail copy of PM2008
-Run Impulse and after it does any updates, click on the KEY icon in the upper right. This will take you to the registrations page.
-Register the copy of PM2008 that you bought, and then you'll be able to go to the My Games tab and use Impulse to install your game. Using Impulse will provide you with all of the latest updates to the game as well.
Post here if you have any trouble.
