Windows 7 was made available this week with massive discounts if customers pre order them, but they could soon run out.
Microsoft announced a programme which will offer £100 off the home user version and over £100 off the professional version.
"They are going fast," said a Microsoft spokesman. "We had a limited amount of copies and demand has been high and we are running out."
Microsoft said the offer was a way of saying thanks to people that helped it develop Windows 7 through feedback.
"Many of you offered feedback that helped us make Windows 7 work the way you want. We think we found the perfect way to thank you a chance to be one of the first (without having to wait in line) to get Windows 7 at a special low price," read a statement on Microsoft's website.