I just happened to have a similar problem. Error message 0x800f082f when trying to upgrade to Vista SP2 through Windows Update

. My problem was that the install was unsuccessful, which resulted in the following situation:
1) Vista worked, but as SP1 in System Info
2) The attempt to uninstall the update led to an error and was unsuccesful
3) Whenever turning on or shutting down the PC I would get a screen with Vista saying it was installing an update, but that would not happen
4) Windows update continued showing an update missing (but with fewer Mb than the original install)
5) sfc /scannow would not work properly any longer
6) ms suggested solutions would not work (
http://support.microsoft.com/kb/947366/)
7) all restore points had vanished
The solution that eventually worked consisted in

:
1) synchronizing the system clock with time.windows.com (see related posts on "rearming" Vista)
2) deleting the PendingRenames folder and its contents in C:\windows\winsxs\temp\
3) deleting the xxxx.xml file from C:\windows\winsxs\ folder
4) rebooting the system until all update install messages disappear
5) uninstalling the update (this time it worked)
6) reinstalling sp2 with the downloaded installer rather than using windows update
Hope this helps anybody experiencing the same issue

.