Often people get confused as to whether it would suffice, if they buy a windows 7 Upgrade product like Windows 7 Home Premium Upgrade, Windows 7 Ultimate Upgrade or a Windows 7 Professional Upgrade. This is because Microsoft offers a upgrade version as well as a full version of the various Windows 7 products.
A windows 7 Upgrade version is usually much lower in price than the full version. The upgrade version is however meant for those PC users who already own a licensed version of windows XP or windows vista. If you don’t have a licensed windows software running on your PC, then you are not entitled to buy and use a windows 7 upgrade product, though you could technically use it to install on a PC that doesn’t have any windows product installed.
Yes, it is technically possible to use an upgrade disc to clean install windows 7 on a PC that doesn’t have any previous version of windows 7 installed on it. In fact, if you are having windows xp on your PC, you will have to do a windows 7 clean or custom installation. Further, if you own a 32-bit windows vista and you intend upgrading to a corresponding 64-bit windows 7 on your PC, then you need to do a clean installation. Other windows users can do what is termed as an in-place upgrade installation of windows 7.
So, microsoft does make it technically possible to install windows 7 on a blank hard drive in a PC, but you may not be legally permitted to do so.If you happen to use an upgrade disc to clean install windows 7 on a PC without being legally eligible i.e. if you don’t have a licensed copy of windows xp or vista running on that PC, then microsoft may not support you if you face any technical issues and you may even find yourselves in legal trouble.
This has been beautifully illustrated by “msmallbiz” on the MSDN blog here.
If you own a PC with a pre-installed windows XP or Vista (Windows OEM license), then you can use the upgrade disc only on such computer and not on any other computer. You will probably understand this better if you read through what a windows 7 OEM license entitles you to.