I am administrator.

Now here's the delio, thanks for the heads up Resinball, I had already made yet another installation of Windows 7 (I always do) on another partition and installed Halo 2 on that.

It worked.

Now I have to tell GFWL about the the exceeded number of usages on my product key (I got a new motherboard on my laptop, I've activated plenty of times before I that. I'm pretty sure that set it off.) Though why it work on that partition of Win7 (1 day ago) and not the one first started (which was installed pretty recently (about 4 days ago))?