If you were trying to install the EXPANSION PACK, then should it be safe to assume you HAVE the main game already installed? If not, there is your problem. Frozen throne is not a stand-alone game. You are required to have Warcraft III already installed before installing Warcaft III: the Frozen Throne expansion pack.
If you DO have Warcraft III installed already and are trying to install the expansion pack, insert the disc for the expansion into your CD/DVD tray, and close it. Go to 'My Computer'. Right-click on the drive that has the disc in it, and choose explore. Look for an executable file in the root directory of the disc named something along the lines of installer. Right-click on that, go to properties, and go to the compatibility tab. There will be a dropdown bar with choices. You want to choose Windows XP (Service Pack 2), and hit ok. Now rightclick on that executable again, and choose run as administrator, and it should work.
If your Warcraft III discs are registered with Blizzard via a battle.net account (Required to register the CD Keys online at installation) you can download the game installer for free. There may be an updated version of the installer that is up to date for Windows 7 there.