The "Proof of Ownership" requirement on a computer that has Norton pre-loaded is a specified print-out from the Help menu. Additionally, I would presuppose your computer's invoice would show the Norton as a pre-installed item
As to buying "Works" for $20, including shipping, that does seem a little "too good to be true" doesn't it? And why bother, if you can buy at Costco with the rebate, and know what you really are buying, and also know you have delivery, and somebody won't scam you on your credit card. But that is all a personal choice.
I would imagine Wall-Mart also sells Norton at similar prices.
The Utility Package I looked at today might well have been Works. I'd suggest that Norton Internet Security could be more valuable in your situation than Works, given the same net price of $25.
Good luck, whatever you decide.
AK
p.s. If you don't update, then simply send Norton an e-mail asking how to disable the pop-up. Who knows, maybe they will send you a coupon or sumptin'?