There's an ongoing problem with .NET Framework 2.0 and 3.5 updates for XP and Server 2003 which causes 3 security updates (KB2518864, KB2572073 and KB2633880) to be repeatedly installed. The problem started some time today (possibly at around midnight EST).

If you have opted to install updates automatically (the default) it's possible the updates are being repeatedly applied in the background. That's not happening with my XP system as I have that option set to "Download updates for me, but let me choose when to install them" (I get a notification that they need to be installed soon after successfully installing them). You can change the option in Start - Settings - Control Panel - Automatic Updates.

According to my update history KB2633880 was included in the February 2012 set of updates, KB2572073 in the October 2011 set and KB2518864 in the June 2011 set. It also shows they've all been successfully installed 5 times today.

There are loads of threads about this on the Microsoft support forums (and elsewhere), including windows update keep installing "kb2518864"/"kb2572073"/"kb2633880" repeatedly

Apparently Microsoft have acknowledged there's a problem and have promised a fix. The 4th reply in the thread I've linked is a sledgehammer solution from a M$ support engineer (it uninstalls all of the .NET Framework versions and reinstalls them from scratch), but I'm not going to take that route unless Microsoft officially announce it can't be fixed in any other way.
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"The ultimate test of a moral society is the kind of world that it leaves to its children." - Dietrich Bonhoeffer

Microsoft have fixed the detection problem which was causing this (see here).
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"The ultimate test of a moral society is the kind of world that it leaves to its children." - Dietrich Bonhoeffer