Just fixed one like that on Vista.

In Vista I had to turn off UAC for a moment. Now I log in as that one user (nobody else logged in) and run up Word and let it finish the installer.

Next time, no installer (at least here.) Then back to UAC to turn it back on.

PS. Yes I bet there are other ways but this was what I used last month.
PPS. And yes, OTHER PROTECTION SOFTWARE may be blocking the installer but that's up to you to kill off for a moment.