Three of the world's largest PC manufacturers are exploiting loopholes in Microsoft licensing terms as a means of supplying Windows XP beyond its official kill date.

Dell, HP and Lenovo have found ways of prolonging XP's lifespan, despite Microsoft's best efforts to phase out the old OS out in favour of Vista, which has thus far been given the cold shoulder by many companies and consumers.

Dell and HP are exploiting a clause in the downgrade rights that come with Vista Ultimate and Vista Business.

"Dell has the ability to exercise 'Windows Vista downgrade rights' on your behalf in the factory if ... you would prefer to have Windows XP Professional preinstalled on your PCs," said Dell on its website.

The loophole has allowed Dell to install Windows XP Professional for free on selected Latitude, OptiPlex and Precision systems aimed at business customers.

Dell will continue to offer XP Pro at a fee for consumers and small businesses buying Vostro and XPS gaming systems.

HP is also using the 'downgrade' loophole to sell XP-based systems until 30 July 2009.

Lenovo, meanwhile, is including an XP recovery disk on some Vista-based systems allowing owners to revert to the older OS if they choose.

The move will come as bad news for Microsoft which recently announced disappointing third-quarter financials, largely attributed to a year-on-year fall in the sales of Windows software.