Instructions
Navigate to Blizzard's Battle.net website on your Internet browser.
Click "Log In" at the top of the page and enter your email address and password. If you do not have a Battle.net account, create one by clicking "Sign Up Now!" at the bottom of the window.
Click "Account" at the top of the page.
Click "Games," located in the bottom right corner. Click "Add a New Game."
Enter your 13-digit "StarCraft" CD-key in the "Game Key" field. Click "Add Game" when finished to add "StarCraft" to the list of games you own. The CD-key is listed on the back or inside of your CD case, or on the back of the game's manual depending on when the game was purchased.
Click the "Windows" icon, next to the newly-created "StarCraft" listing on the "Games" page, to download the installer. Select an easy to remember location to download the file to, such as your desktop.
Double-click the installation .exe file to begin installing the game. All files for the game are downloaded using the installer -- an Internet connection is required at all times during the installation. The latest "StarCraft" patch is also downloaded when installing the game online.
Double-click the "StarCraft" icon on your desktop once installation is complete to launch the game.