ANSWER: defrag the database using esentutl.exe
Here's what I did
C:\Windows\system32>esentutl /d C:\Windows\SoftwareDistribution\DataStore\DataStore.edb
Extensible Storage Engine Utilities for Microsoft(R) Windows(R)
Version 6.0
Copyright (C) Microsoft Corporation. All Rights Reserved.
Initiating DEFRAGMENTATION mode...
Database: C:\Windows\SoftwareDistribution\DataStore\DataStore.edb
Temp. Database: TEMPDFRG4872.EDB
Defragmentation Status (% complete)
0 10 20 30 40 50 60 70 80 90 100
|----|----|----|----|----|----|----|----|----|----|
...................................................
Moving 'TEMPDFRG4872.EDB' to 'C:\Windows\SoftwareDistribution\DataStore\DataStore.edb'..
Note:
It is recommended that you immediately perform a full backup
of this database. If you restore a backup made before the
defragmentation, the database will be rolled back to the state
it was in at the time of that backup.
Operation completed successfully in 27.875 seconds.
Now i have an extra 400MB free - makes the difference on vista!