Upgrading Windows Vista online from the existing disc is now possible because of Windows Anytime Upgrade feature that comes bundled with Vista.
To check it out :
1.Go to Control Panel>System and Maintenance>Welcome Center.
2.To upgrade to another version of Windows Vista, select Control Panel >System and Maintenance >System, and click Upgrade Windows Vista
3.Of course,the upgrade is not free 
so you will need to pay online,the online upgrade link will open when you click on Upgrade Windows Vista.
4.Choose the upgrade that is suitable,the existing system will be upgraded or you can order a seperate disc by following the online instructions for the same.
Peace.
