Vista Updates Issue

I have downloaded and successfully installed the following update 9 times:

Security Update for Microsoft XML Core Services 4.0 Service Pack 2 (KB936181)

Unfortunately whenever I reboot or leave the computer on for longer than
about an hour, I am told that I need to install a new important update, which
just happens to be the one I just installed. I'd like to know if there is
any way to fix this issue, because frankly I need to have the latest security
updates, I do a lot of web browsing and downloading. If anyone here has any
information on getting the updates to work again, please let me know.

I haven't tried system restore yet, because I installed a good deal of
software on this computer the day that the issue occured. Before you say
it's the software, you should know it was Microsoft Office 2007, Cain,
DaemonTools, some software from Microchip Industries for robotics coding,
Nero 8, and WireShark. If you HONESTLY think that this software is what did
it, let me know, but I don't think any of these applications would cause
system update failures.