Windows 8 Pro upgrade is available for $39.99 till January 31st 2013, as a promotional offer. If you qualify for the Windows Upgrade Offer, then you can get it for $14.99 USD. After the date, Windows 8 Pro upgrade will be available for $199.99, and the standard version of Windows 8 upgrade will cost you $119.99. So if you are interested in upgrading to Windows 8, it's the only time!
All the Windows based PC qualify for the upgrade, whether they are running Windows XP, Windows Vista, or Windows 7. You can even upgrade your pirated version of Windows 7 to Windows 8 Pro. It's your final time go legit, if you haven't yet done so.
If you prefer to do a clean install instead of upgrading, don't worry, you can burn a DVD and then perform a clean installation with the upgrade licence. You just need to be on Windows base PC when buying Windows 8 Pro upgrade licence via the Windows 8 Upgrade Assistant. It's a small application which will be needed while upgrading.
If you are confused whether to upgrade to Windows 8 or not, then you can download the 90-day trial of Windows 8 and test it on VMWare or VirtualBox. But hurry up to available the opportunity as the $39.99 promotional offer is going to lapse within 10 days.
After buying the Windows 8 Pro licence, don't forget to get your free Windows 8 Media Center Pack key. It costs $9.99 but Microsoft is giving it away for free till January 31st.
So hurry now, lest you should lose the opportunity!
Update: The price of Windows 8 Pro upgrade has been raised to $199.99 but if you are a student, you can still get Windows 8 Pro for $69.99.