Microsoft has extended Windows XP's availability for another six months. Large computer manufacturers will be able to "downgrade" machines from Windows Vista Business or Vista Ultimate to Windows XP Professional until the end of July 2009, said the company.

Microsoft had planned to halt XP Professional media shipments to major computer makers after 31 January 2009. These will now continue for another six months.

"As more customers make the move to Windows Vista, we want to make sure that they are making that transition with confidence and that it is as smooth as possible. Providing downgrade media for a few more months is part of that commitment," a Microsoft representative said.

Smaller companies – known as "system builders" - will not be allowed to purchase Windows XP licences to install on the machines after 31 January, the company said.

Some PC makers have long claimed that they would provide XP downgrades on new computers past the 31 January deadline. Last June, for example, Hewlett-Packard talked of a July 2009 cut-off. "HP... will continue to offer this option on its business systems through at least July 30, 2009," a company spokesman said.

Microsoft began phasing out Windows XP when in June it stopped retailing the operating system, withdrew Windows XP Home from use on new PCs and allowed XP Professional to be installed as a Vista downgrade.

The latter tactic takes advantage of Vista's end-user licensing agreement (EULA), which allows users - and in their stead, computer makers - to install Windows XP Professional while also providing media for Vista for a possible upgrade later.

More than a third of all new PCs are being downgraded to Windows XP, according to data from a US company that operates a community-based performance testing network.

It's also possible that XP will be widely available long after 31 July, 2009. "Downgrade rights do not expire," Microsoft's spokeswoman said Friday.