Would that explain why I'm getting a pop-up message as a "limited user" whenever I try to launch OBMM?
Probably. You can tell Oblivion to run as an Administrator to get around that kind of thing I believe, or disable UAC. But it's best, and most secure to just install outside of C:\Program Files. Reason for this is that Windows sees that directory as protected and won't allow changes to it without confirmation or elevated rights. So, for example, if you try to make a mod and save the ESP file...first time it'll work, but if you try to edit it later it will be locked because you're now making a change. (That's what I've heard, at least. Never ran into the problem myself, as I tend to use a secondary (mirrored) drive for games I mod.)
I'm running Windows 7 (x64) with Oblivion installed on a secondary drive and it works perfectly, with and without mods.