If you've purchased a Windows 7 PC between June 2, 2012 and January 31, 2013 you are eligible for a discounted upgrade to Windows 8 Pro. When it releases you'll be able to upgrade your relatively new PC for only $14.99.
In order to take advantage of the Windows 8 upgrade offer you'll need to register online over at the Windows Upgrade Offer website. Up until today the site was simply a "Coming Soon" placard but today the site opened up to accept registration from eligible users.
The process is simple. Head over WindowsUpgradeOffer.com and choose your Country. From there enter your contact information, computer information and when the upgrade is available you will receive an email with instructions on how to upgrade your Windows 7 PC to Windows 8 Pro.
Now if you're running a Windows 7 PC not purchased during the discounted period, you're not in too bad a shape. The full price to upgrade a Windows 7 computer to Windows 8 is $39.99. The Windows 8 launch date is still set for October 26th so we should start seeing more details on upgrading in the coming weeks.