Dave is correct. I've just carried out a Google search and the issues with SP3 have not been resolved. Some (like Simon) get away with it whereas others end up with a computer that re-boots itself in an endless loop. 
Ah, I thought that only happened with a certain type of AMD machine, but as mine is Intel, I think I was fairly safe.
So you have it then Simon did it come in your updates or have you downloaded it from Microsoft?
I used Windows Updates to get it, Kaz, but as Clive said, if you have an AMD machine, you may want to hold fire on it.