Microsoft's decision last week to delay the end of Windows XP sales five months means users have just that much longer to jilt Vista and return to the older -- and some say more mature -- operating system. But even with XP's reprieve, few PCs come with anything but Vista. Even if you wanted to take advantage of the wider window of XP opportunity, you might not know where to start.

Corporations that acquire Microsoft operating system software through volume license agreements such as Select License, or who have signed up for Software Assurance, can downgrade any Windows software at any time. For the rest of us, just 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 lets owners of some versions of Vista replace it with Windows XP without having to pay for another license. In effect, the license for Vista is transferred to XP. Think of it as a swap, Vista for XP, not as an extra license. 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.

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 like Flynn.

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. You are, as they say, SOL.

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.

Where do I get the XP install disc?

Until this summer, Microsoft put the responsibility on the end users' shoulders. For example, in this Vista downgrade rights document targeting resellers, the company said "media is provided by the customer."

A few months ago, however, Microsoft relaxed, and began allowing resellers to provide Windows XP setup CDs to customers buying Vista Business- and Ultimate-equipped PCs. In some cases, discs are shipped with the PCs; in others, users must request them. Don't bother calling Microsoft; it won't provide installation media, and will instead direct you to your reseller.

If the computer maker won't send a Windows XP Pro disc, you're on your own. While perhaps not easy, getting your hands on the install media isn't impossible. Any copy of Windows XP Professional will do -- it doesn't matter if it's already been installed and/or whether the license is in use -- as long as you can find its product key. Install it (see "How do I downgrade?" above) using that key, then activate over the phone with the Vista key.