Microsoft confirmed yesterday that the new Office 2013 will not run on older PCs powered by Windows XP or Vista.

"The new Office will work with Windows 7 and Windows 8," a Microsoft spokesperson said Monday in an email reply to questions about Office 2013 and Office 365. "Vista or XP will not support the new Office."

Users running those operating systems will not be able to advance beyond Office 2010, the suite that launched about seven months after Windows 7.

The omission of the two operating systems means that more than half of all Windows computers -- 54.6%, to be exact -- now in place will be locked out of the upgrade, according to statistics compiled by Web metrics company Net Applications.

Windows 7 does not yet have a majority of all Windows PCs: In June, it powered 45.1% of all PCs that went online, said Net Applications. Windows 8, which has not been released except in preview formats, accounted for just 0.2% of all copies of Windows.

But those shares are moving targets, and will be different come the release date of Office 2013 and the various Office 365 subscription programs. It's difficult to project how much more share Windows 7 and Windows 8 will have then, since uptake on Windows 8 would be a blind guess, and no one knows whether Windows 8's gains will come from former XP or Vista users, or from people moving up one edition from Windows 7.