"Windows did not pass genuine validation"

I've been using Vista Home on a Model No. Dimension C521 for years.  All at once I get a message "Windows did not pass genuine validation.  Security Essentials will become disabled in 30 days if you do not resolve this issue.  To continue using Security Essentials, click "Go online and resolve now and get genuine windows.  Link: Go online and resolve now."   http://www.microsoft.com/genuine/validate/ValidationResult.aspx?displaylang=en&cCode=USA&PartnerID=258&PageName=validate 

When I go to the link, the site tells me that Windows Genuine Advantage has to be installed.  Clicking on "Install" installs WGA, but then it says the validation process was interrupted and tries to make me upgrade to Windows 7.

I can not afford to upgrade at the present time.  What can I do?