The patch, which we wrote about here, has been causing grief for many users. If you have Windows Update running on automatic and you're experiencing a nasty slowdown on your system, you might want to try removing patch 811493. Users with the performance problems report that removing the patch fixes things.
The patch fixes a relatively minor security problem where an attacker would have to have physical access to the machine.
A discussion about the problem, if it is one, has started on Usenet, here. ยต