Microsoft has started taking pre-orders for the its desktop OS Windows 8.1 ahead of the global launch on 17 October.
, anyone running Windows 8 will automatically be upgraded to Windows 8.1 for free, while everyone else running earlier versions of the Microsoft OS will need to pay $119.99 or $199.99 for Pro.
The pre-order campaign, opened today in the US, is aimed chiefly at customers running Windows 7, since as Microsoft notes: "Windows 8.1 is not designed for installation on devices running Windows XP or Windows Vista."
Microsoft says that files can be transferred easily during the upgrade from Windows 7 to Windows 8.1, but as it, desktop apps including Office will need to be reinstalled after users have upgraded to the new OS.
Meanwhile, Microsoft is encouraging anyone running Windows Vista or XP to upgrade to Windows 8. Users that upgrade to Windows 8 now will "qualifiy" for the free upgrade to 8.1.
It's got some considerable ground to make up: it's thought 500 million machines around the world, despite Microsoft .
ZDNet has asked Microsoft whether the pre-sale campaign will be extended to non-US markets and will update the story if it receives one.