MSXML 4.0 SP2 Security Update (KB927978)

Hi, I have an issue with the "MSXML 4.0 SP2 Security Update (KB927978)", the problem is that it keeps insisting that I install it time after time. This is despite the fact that each time I install it,  once completed it says that it has sucessfully installed it's self and that if i look in "add/remove programs it is listed as installed?! The only way I can get it to leave me alone is to click on the yellow sheild and ask it to stop telling me about it, why is this??