Since this is probably the most annoying thing Microsoft has ever put into their operating systems, here's some links to sites that teach you how to disable it:

Download everything from Microsoft without WGA Check

Disable and Remove Windows Genuine Advantage Notifications Nag Screen

For those of you in a hurry, here's the quick answer:

1. Download mgadiag.exe
2. Start mgadiag.exe and copy the download center code
3. Visit a download page at microsoft.com
4. Append &Hash="download center code" to the url (example &Hash=6VJPCR9), no quotation marks needed
5. Hit enter