I bought the download version through gamestop and found that it had finished downloading in the morning. When I attempt to install, the installer fails when extracting files at the very beginning. The last one to extract is infinist.exe.

Then I get the error message that the main setup application could not be launched.

I am currently running Windows XP SP2

For lack of a better insight, I am re-downloading the game from the Gamestop database.

That doesn't sound like any executable we make (unless its some weird internal part of NSIS, but I've never seen it before) so it sounds like this is part of the GS downloader system.

Since you bought a digital download, has Gamestop given you your serial number yet? If they have, then I believe you can register your account here (which you'll have to do anyway) and then simply download the file directly from Cryptic. Unless I completely fail to understand the system involved, you don't have to use the version from a digital retailer (and given some of the issues I've heard with patching and updates, it's quite possible that you don't really *want* to use a retailer's version.)

The installer, and instructions on how to use it, are at:
http://www.startrekonline.com/download

If you still can't get the Gamestop file to download and decrypt properly, you might give that a try.

Edit: Also, you said you're running XP SP2 - you might need to consider installing SP3, and the updates to Internet Explorer. As I understand it, the launcher is essentially a script that runs through IE, and things get weird if you're not using the current version.