Screenshot by Lance Whitney/CNET

Those of you who recently bought a Windows 7 PC can score Windows 8 for just $15, but you'll have to register by the end of the day Thursday.

To drum up interest in its latest OS, Microsoft cooked up a couple of upgrade deals last year.

The first one allowed any user of Windows 7, Vista, or XP to upgrade to Windows 8 for $40. That deal ended January 31.

The second one lets people who bought a Windows 7 PC between June 2 of last year and January 31 of this year get Windows 8 Pro for just $15. Though that deal expired last month, you can still register for it, but you'll have to act by this Thursday, February 28.

The registration process prompts you for your contact information as well as the brand and model of your Windows 7 computer, the retailer, and the date on which you bought it. The process may also ask for your Windows 7 license key.

Assuming your PC checks out, you then receive an e-mail with a promo code and a link for the Windows 8 upgrade. The link downloads the Windows 8 Upgrade Assistant, which you run on your Windows 7 PC to pay for the product and install the software.

An FAQ provides more details on the Windows 8 upgrade offer. People who bump into problems with the process can contact Microsoft customer support via the upgrade offer support page.

What if you want to take advantage of the $15 deal but don't want to upgrade to Windows 8 right now?

You can download the upgrade and simply store the installation files on a USB stick or burn them onto a DVD from an ISO file. You can then install the upgrade anytime you want and on any PC you want. One of my previous articles explains the entire process for creating the Windows 8 installation media.

When exactly does the registration expire? A Microsoft representative told CNET that the registration on the Web site and the promo code redemption will continue to work through February 28 worldwide. That gives people until the stroke of midnight on Friday to complete the process.

Following the cutoff, Windows 8 jumps in price for virtually everyone -- $120 for the regular edition and $200 for the Pro version. Only students will still be able to score a discount.

So even if you're on the fence about the new OS, those of you who qualify for the $15 deal may want to register for it while you still have time.

Update, 11:40 a.m. PT: Adds exact expiration time.