Yes, you can still buy a new PC with Windows 7 installed instead of Windows 8. While Microsoft is also still offering a way for those users to upgrade to Windows 8 Pro for just $14.99 until January 31, 2013, it has apparently added a new requirement for that to happen.
When Microsoft first started this effort back in August, all people had to do to qualify for this offer was to go to the Windows 8 Pro upgrade page and type in the PC model name and maker, and the date which you bought the PC. Now CNet reports, and we have confirmed, that Microsoft is also requiring users to type in their unique Windows 7 product ID in order to get the $14.99 upgrade.
This change appears to be quite recent but so far Microsoft has not commented publicly on why they added this additional requirement. It's possible that the company wanted some extra proof that the people who bought a new Windows 7 PC did so during the required time period. Microsoft is also offering a $39.99 upgrade to all older Windows 7, Vista and XP PCs to Windows 8 but that price will also end on January 31.
Source: CNet.com | Image via Microsoft