First, I would not recommend installing 811493 (or install it, and if you notice a system slowdown, remove it from Add/remove Programs in the Control Panel instead of doing a system restore).For get Windows Update to work again, renaming or deleting the Catroot2 folder may solve the problem.
Go to http://support.microsoft.com/?scid=kb;en-us;326815
Follow the steps 6 to 8 in the Resolution section of above link (do a reboot first so the computer is "fresh" before you do the steps).
Then try to install the update again.
Disregard that the link above is for SP1, the Catroot2 problem is relevant for other updates as well. The Catroot2 problem can be avoided by installing Q817287 (the one you have problems installing ;-)
Windows Update 643 Error and the Catalog Database http://support.microsoft.com/default.aspx?scid=kb;en-us;817287
If renaming or deleting the Catroot2 folder doesn't help, try this:
Check your "windows update.log" file for specific error code (the log file is in your windows folder) and take look at this Web page:
Windows Update Troubleshooter http://v4.windowsupdate.microsoft.com/troubleshoot/
In my case the initial update failed. Subsequent attempts would bomb at 0%. I followed the steps 6 through 8 above to rename the folder, and after rebooting I went to the Update window and accepted only the Q817287 update. I haven't tried the Q811493 update since, and am unlikely to try!