By now you will have read the 'long live Windows XP' headlines, with Microsoft giving the operating system a stay of execution by allowing users to buy it for five months longer than previously anticipated. The decision has been labelled as another black mark against Windows Vista, which has failed to meet Microsoft's sales expectations. Some Vista users are even reverting back to XP, while others in PC Advisor's Windows Vista forum claim they'd like to, given the right know-how.

So what does it take to turn back the clock? Read on for more about the trip to Windows yesteryear.

What is a downgrade?

To Microsoft, 'downgrade' describes the licensing rights it grants to older operating systems. Downgrade doesn't mean the process for rolling back Windows from Vista to XP, since there isn't such a procedure, not in the generally accepted use of 'upgrade'. In an older-to-newer move, developers usually make it possible to retain all the digital detritus on the drive, from already-installed applications and Word documents to iTunes tracks and family photos, while updating the system files. Not so in a downgrade.

Specifically, these downgrade rights let owners of some versions of Vista replace it with XP without having to pay for another licence. In effect, the licence for Vista is transferred to XP. Think of it as a swap, Vista for XP, not as an extra licence. By Microsoft's end-user licensing agreement (EULA), you can't have both the Vista and its downgraded XP installed at the same time on the same or different machines. You have to pick: It's one or the other.

See also: First look: Windows Vista SP1

To the vast bulk of users, though, 'downgrade' is a synonym for reverting to an older version. In that case, it simply means dumping Vista and returning to XP.

So, what downgrades does Microsoft allow? Owners of the OEM editions of Vista Business and Vista Ultimate can downgrade to Windows XP Professional, including Tablet PC Edition and x64 Edition. Only the OEM editions qualify for a downgrade, so if you purchased a new PC with either Business or Ultimate preinstalled, you're in luck.

Those who aren't: all users of Vista Home Basic and Vista Home Premium, and anyone who upgraded to Vista using a retail edition of any of the operating system's SKUs.

How do I downgrade?

Install a copy of Windows XP Professional with the product key that came with the copy, and then when you hit the activation screen - which is near the end of the installation process - select the activate by phone option rather than the online method. You'll likely end up talking with a live rep; tell him that you're downgrading from Vista to XP, and give him the Vista product key. The rep is supposed to walk you through the rest.

See also: 75 free ways to refresh Windows XP & Vista