I recently performed a repair install of WinXP SP2 on a machine that was current on updates. After the repair install, the Windsows Update Site showed that the machine needed 80+ updates, but all attempts to install updates failed. I did a lot of searching for a solution with no luck. Finally, using the troubleshooting link given by the update site when the installation failed, I found a Microsoft document that indicated a repair install can result in a number of dll files being overwritten and listed a series of commands to run from a command prompt to correct the problem. I ran the commands and all was well. Sorry, I didn't save the info, but you should be able to find it through the troubleshooting link on the update error page.
