Why won't Age of Empires III: WarChiefs install?

I have AoE 3 installed already, so I don't understand the problem. When I try to install Warchiefs it tells me that AoE 3 wasn't installed on Allusers or this user. The thing is, when installing AoE 3 I selected all users, and it wouldn't even matter if I did or not, there's only one user account on this computer. I don't understand why it's doing this. Thank you in advance