seems like it's trying to finish installing an installation that you once started.
have you tried un-installing Office from the control pannel? it seems like the un-install didn't happen properly and thus everything is a little buggered up..
this tells you why the problem happens.
(note that it probably happens when you are starting windows or explorer because it's trying to load office pluggins (and the fact that they arn't installed means it can't, and so wants to install them).
I suggest you use a program like Hijack this to see what is supposed to be starting up when your computer starts and remove and office related things, (included office related IE pluggings that are causeing the installer to run when you launch IE.