Because that means before Microsoft gets to Windows XP Service Pack II, we'll have Service Pack 1a, 1b, 1c, 1d [that's enough letters of the alphabet, Ed.]
But long before we get to Microsoft Windows XP Service Pack 1z, we have to cope with Microsoft Windows XP Service Pack 1a.
It's all to do with Sun Microsystems and Java, as the Vole had a court order "stayed" yesterday in the US.
This particular "a" download will be smaller than 30MB, as you can read on the Vole's site, here. µ