Microsoft announced on Monday that it will allow users of Windows XP, Vista, or 7 to upgrade to Windows 8 Pro for only $40 as part of a special promotion when the new OS becomes generally available later this year. This is, by far, the lowest upgrade pricing Microsoft has ever offered for Windows, and an exceptional value.

But you’ll need to act fast, as the promotional upgrade pricing will be in effect for a short time: It starts at the general availability of Windows 8 and last through January 31, 2013.

“We’ve continued to listen to our customers and have expanded the ability to download Windows 8 to over 100 countries and 37 languages, a post on Microsoft’s Blogging Windows site notes. “We have simplified the Windows upgrade experience with the Windows 8 Upgrade Assistant which supports you during your upgrade with everything from selecting your language to pausing your download to built-in compatibility checks - it’s seamless. And if you’re an enthusiast, you will have the flexibility to download and control how you upgrade.”

The $40 pricing applies to electronically downloaded versions of the software that are installed via Windows 8’s new web-based installer at Windows.com. (Coincidentally, I just wrote about using the web-based installer on Monday as well.) If you prefer retail packaging and a physical disc, you can obtain the Windows 8 Pro upgrade during this promotion for $70.

Additionally, Microsoft announced that those who upgrade to Windows 8 Pro through this special promotion will be able to purchase the Windows Media Center add-in pack for free during the promotional period as well. Microsoft previously said that Media Center would be made available to Pro users for a small, as-yet-unannounced fee.