There are 3 ways.
1. Simply uninstall it from the old machine using Control Panel > Programs > Uninstall a Program (Add or Remove Programs in XP) - that will remove one of the licenses online.
If that isn't possible
2. Go to your online account and deactivate the license there. http://home.mcafee.com/root/MyAccount.aspx click on the blue Subscription link then the Deactivate button. It helps if you've given each machine a nickname. (Click image to enlarge)
3. You can do it the easy way and phone Customer Service via the link under Useful Links above, it's a free call.
That was a very quick reply!
I shall do that
OK all the best ;-)