Raul,
Many thanks, the pop up is the standard windows installer asking for disk 1
(actually its about 10 pop ups of exactly the same type)
I've followed your instructions and the results were
Outlook /safe = no reaction
msiexec reg etc. = no reaction that I can see
MSS$MICROSOFTBCM is started and set to automatic
The error turns up only when I load another peice of software so I'll need
to wait for a while until I know the result.