On modern Windows servers, you can create a self-signed certificate with powershell, using the new-selfsignedcertificate command. By default, this certificate will only last for 1 year before expiring. To create a certificate that lasts a little longer, fire up an admin powershell, and do this:

$fromtoday = get-date
$5years = $fromtoday.addyears(5)
new-selfsignedcertificate -dnsname mycertname.mydomainname.com -notafter $5years -certstorelocation cert:\localmachine\my