Microsoft: We're in yur drive, hacking yur system
 

The guys and gals over in Redmond have been busy little beavers
for the last few weeks. It has apparently been confirmed that Microsoft
has been "stealth patching" some NT based computers without the
knowledge or consent of the user (this includes computers with XP and
Vista on them) even if the user asked to be notified of updates.
Basically, Microsoft has been placing software components on user
systems (yours likely included) remotely and secretly (ok - "not very
transparently") - regardless of whether you want the update or not.

http://www.hardwarelogic.com/Staff-Blogs/285.html