Instructions
Open the box in which your Ghostbusters game was packaged. Inspect the case to find the Atari serial number. You should find it printed on a sticker on the inside.
Open the email you received if you purchased the game online. The message should contain your receipt, which should show your serial number. If it doesn't, or if you can't find the number, contact the company from which you downloaded it and request that they send it again.
Download a program such as Magical Jelly Bean's Keyfinder or License Crawler. Run the program to scan your computer for installed serial numbers. Keep in mind that these applications only work if you've already installed and activated the Ghostbusters game on your computer.