You can tell it not to inform you of it again, and it should leave you alone.
I told Windows Update not to show WGA to me anymore, along with a couple other updates I don't need. The other day it came up saying something to the intent of "You've hidden a critical update for your computer. I'll show it to you so you can unhide and install this critical update."
Microsoft makes it look like not installing this will blow up your computer (it won't -- yet), and they're pushing it far harder than they've pushed critical updates for gaping security holes like WMV.