Thanks for the advice guys.
For the record, the re-formatted Pro machine is doing fine even with auto updates activated. This box has SP3 too.
On the machine running Home, deleting the DataStore and then SoftwareDistribution (c/w reboots) did not correct the problem so as I find the time I will do a re-register of the dll's. For now, updates are de-activated.
This is like a virus! Now we have another of the Pro boxes is afflicted. If I had the confidence I'd start switching to Linux.
STL