proplus.msi is the installation file used to install all office
PROfessional PLUS (yes ProPlus) services, hence Microsoft Service
Installation (MSI). But IT DOES NOT COPY ITSELF, therefore everytime you
upgrade or re-install the product the proplus.msi will ask for this file.
Bottom line: the system wants the Office disk.
With Office, you have to have the original cd (or the files off of it) to
install the service packs. Just put the original office CD in the drive,
then the update will work fine.