Microsoft Corp opened its Windows 8 operating system for pre-orders on Friday, setting the price for an upgrade to the full version of the software at $70 for a DVD pack.

Users can also wait for launch on October 26 to download the system onto their computers for $40, an offer price that will expire at the end of January. PCs running Windows XP, Vista and Windows 7 will be able to upgrade to Windows 8.

Shoppers can reserve the software pack at Microsoft's own stores, Amazon.com, Best Buy, Staples and elsewhere. Microsoft has not yet announced the price of the full software to install from scratch, as opposed to the upgrade. The current price for a comparable version of Windows 7 is $200.

Any customer who buys, or already bought, a Windows 7 PC between June 2 and the end of January 2013 will be able to get an upgrade to Windows 8 Pro for $15, a move designed to prevent a drop-off in PC sales before the launch of Windows 8.

Microsoft also said PC makers such as Acer, Asustek, Dell, HP, Samsung and Sony were also now taking pre-orders for machines with Windows 8 pre-installed.

The world's largest software company did not mention its own Surface tablet PC, which is expected on the market at the same time as Windows 8. Microsoft has not revealed the price of the product it hopes will challenge Apple Inc's iPad.