Acer is offering a free upgrade to Windows 8 on some ultrabook models, saying it will refund the upgrade fee being charged by Microsoft on certain Windows 7 PC models purchased by buyers.

Acer will refund consumers who purchase the Aspire S3, Aspire S5, Aspire M3 and Aspire M5 ultrabook models between June 2 this year and Jan. 31 next year. Users will have to pay Microsoft for the upgrade, but will then be refunded by Acer via a claim website.

The offer is valid in 36 countries in Europe, the Middle East and Africa, but not in the U.S., Canada or other Asian countries. Acer representatives did not immediately respond to requests for comment on whether the offer would be available in other countries.

Microsoft is charging customers US$14.99 for users to upgrade from Windows 7 to Windows 8 Pro on qualified PC models. The fee varies based on region -- for example, Microsoft is charging 14.99 (pounds) US$23 to U.K. customers for the Windows 8 upgrades. PC makers are certifying computers as being upgradeable to Windows 8.

Microsoft has also announced a separate upgrade program in which existing PCs running Windows XP, Vista or 7 OSes can be upgraded to Windows 8 for $39.99. Those PCs do not qualify for the Acer offer.

No other PC maker has announced such a program yet, and Acer's offer could be a small incentive for users to buy its PCs over rival brands.