So I recently got a shiny new custom computer (running Windows 7) from my friend, and because I didn't want to piss my parents off, or spend hours reinstalling everything, I got him to take out the hard drive from my old Windows XP computer, which had all of my sims stuff on it, and put it in the new computer as a secondary (I think) hard drive.

The core game files are all there, as well as the launching applications, but I can't seem to run the game off of the drive. I'm using the compatibility mode feature, and I've run other games off of the secondary drive with almost no issues, so I don't know what the matter is.

I really don't want to have to install it on this computer again, since I've heard it's a pain getting the patches to work with Windows 7.

What should I do?

You need to have the correct registry entries to run the game. The usual way to get the correct registry entries is to install the game.