In the end I gave up trying to solve it. My setting was the same as yours and the system notified me at the correct times when updates were available, and they downloaded and installed without problem when I gave the go-ahead.
Vista and Windows 7 both behave a little better with this and I haven't seen the problem you describe with my newer Windows 7 system.
If your system is otherwise working well and you are able to download the updates and install them as and when they are offered, then it won't do any harm to ignore that warning, even though it is a nuisance.
One thought though and I can't check at the moment, can you check your Updates history on XP? If so, are there any updates that have failed to install recently? It's a long shot, but perhaps the system is confused with one that didn't install properly.
I am getting a windows security alert in the notification area saying that auto updates is turned off. In reality it is set to notify me but don't download. When I click turn on auto updates it says that it can't and that I should go through the control panel -system -auto updates tab. I can also click on auto updates at the bottom of the security center screen. I changed the setting to automatic but the alert is still there. Rebooting didn't help.