What exactly IS the error message you're getting? That's probably the most important thing.

Oh, and while I'm thinking of it...try dragging the installer you're having trouble with to your desktop to install it. Just to humor me ;o). Do the compatibility thing with it first. I'm curious to see if it's because it's trying to install from an external. Not that it should really matter, but stranger things have happened. Just a guess, but that could be why your choices and stuff were grayed out when you went to adjust the compatibility.

Laurie