There are two parts to this question.
1. Can I
purchase an upgrade to Windows 7?
If you have a PC running a full version of Windows XP or Windows Vista then yes you can purchase an upgrade to Windows 7.
The Windows 7 installer will ask for the product key from your Windows XP or Windows Vista machine for proof or purchase.
More info here:
http://www.microsoft.com/windows/buy/offers/upgrade.aspx
2. Can I
perform an upgrade to Windows 7?
This is where it gets a little complicated.
If you are running
Windows XP, Windows 7 Beta or Windows 7 RC then you will not be able to perform an upgrade to Windows 7.
Windows XP and Windows 7 Beta/RC users will need to backup their files and settings (ideally using the
Windows 7 Easy Transfer Wizard) and perform a clean install of Windows 7.
Windows Vista users who want to upgrade to an equivalent or greater version of Windows 7 can do so providing they are not changing from 64bit to 32bit versions of the OS.
This chart should help illustrate your options.
More Info on Upgrading:
http://www.microsoft.com/windows/buy/offers/upgrade.aspx
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