EDIT: Just visited Windows.com. The Buy Windows Landing page has a offer to upgrade any Windows PC (XP and above) to Windows 8 Pro for 24.99 Pounds.
Please Ignore the rest of the post
As per the site provided by AbhishekGirish, , the Windows Upgrade offer is only available for users with valid Windows 7 license (that too in new Windows 7 PCs). Since you had Windows Vista license, it is not applicable for you.
The offer is for customers (e.g. home users, students, and
enthusiasts) who purchase a qualified PC. A qualified PC is a new PC
purchased during the promotional period with a valid Windows 7 OEM
Certificate of Authenticity and product key for, and pre-installed
Windows 7 Home Basic;
Windows 7 Home Premium;
Windows 7 Professional; or
Windows 7 Ultimate.
The promotional price is limited to one upgrade offer per qualified PC
purchased, and a maximum limit of five upgrade offers per customer.
You must purchase a qualified Windows 7 PC between June 2, 2012 and
January 31, 2013 to be eligible for this offer. The last day to
register and order your Windows 8 upgrade is February 28, 2013.
However, there is twist in the end saying that once you have got the key, it is valid for upgrade of any PC with valid Windows XP, Vista or Windows 7 license.
Each upgrade license will apply to only one PC and may only be
installed on PCs with a valid base license to a qualifying operating
system (i.e., Windows XP SP3, Windows Vista or Windows 7)
The FAQ answers most questions you asked:
If I purchased a qualifying PC with Windows 7 Professional on it during the promotion period, but downgraded to Windows XP SP3, does the PC still qualify for the Windows Upgrade Offer?
Yes. The Windows
version on the Certificate of Authenticity and related product key are
what is relevant for eligibility, not the version of Windows being
used. Note that the upgrade experience may vary based on which version
of Windows is installed on the PC.
What edition of Windows 8 will I receive when I upgrade?
A download of Windows 8 Pro (Upgrade)
Do I have to install the Windows 8 upgrade on the eligible PC, or can I install it on another PC?
The upgrade offer is tied to a
Windows 7 PC purchased from June 2nd, 2012 through January 31, 2013,
but you can choose to redeem the offer and install the upgrade on any
compatible Windows-based PC with a qualifying base operating system.
Finally the main question. You will need to UPGRADE a Windows installation as per license agreement for the upgrade offer.
Does the upgrade need to be done on a PC with Windows already installed?
The license terms for the upgrade require upgrading from
a valid "genuine" version of Windows. Visit
http://www.microsoft.com/genuine/ if you are not certain whether or
not your PC has genuine Windows.