As a last resort, buy a copy. This isn't a downgrade, not as Microsoft defines it, but it's what most users think of when they use the term.
What can I do if I don't have downgrade rights?
Nothing is stopping you from punting to XP other than the money invested in the Vista license already on the PC and what it will cost to replace it. The total may be inconsequential to some, a deal-breaker for others. But there are options.
Because you're paying for the swap, you can switch to any flavor of XP. Windows XP Home, for instance, typically sells online for between US$50 and US$90 less than Professional. Windows XP Media Center is usually priced between the two.
Once you pick an edition, you can choose from OEM, upgrade and full product versions, which are priced in that order, lowest first. OEM, often called "system builder," omits support and can only be installed on one machine, ever. Windows XP Home OEM is sold online at for around US$90. The upgrade version, which runs about US$100, can be installed, removed and installed on another PC, but requires proof that you own a legitimate copy of an older operating system. You don't need to install that predecessor, only insert its CD at some point during the XP installation. Eligible versions for an XP upgrade include Windows 98, Windows 98 SE and Windows Millennium.
Finally, there's the most expensive option: the full edition, which sells for around $190. No earlier Windows version is necessary to install this, and like the upgrade, it can be transferred later to another PC.
Of course, the most affordable downgrade is one using the XP installation CD you saved when you upgraded that well-worn machine of yours to Vista earlier this year. You did save it, right? If you didn't get an install disc with that box when you bought it -- and some vendors don't bother, instead slapping restore files in a hidden partition on the hard drive, which has been, of course, copied over by Vista -- you may be able to pry one from the reseller. Dell owners, for instance, can use an online form to request one free copy of the install CD.
I have XP and I'm ready to downgrade. Now what? From here, a downgrade is just like any clean install. You'll need to back up data files, record and/or copy settings and passwords, and make sure you have installation files and/or discs for the applications you'll reinstall in XP. If you've upgraded to software suitable for Vista, it's likely that the newer programs will also run under XP. Copying data and the application installation files you've downloaded from the Web is easiest if you plug in an external drive.
Set the PC's BIOS to boot from the CD/DVD drive -- check your computer's documentation or the manufacturer's Web site for details on this -- and reformat the primary drive. Install XP, then reinstall all applications and copy data and settings from the external drive.
There aren't any downgrade utilities to do the kind of work that upgrade, or migration tools, provide when you're moving up in the world, operating system-wise, so don't bother looking for them. Pity.
Although Vista has been out for less than a year, that's plenty of time for change. If you bought a machine preinstalled with Vista, make sure there are XP drivers for the PC, its components and any new peripherals before you downgrade. Check the computer maker's site. If you find any major holes, reconsider.
I'm lazy but still want to downgrade. What are my options?
If you're fed up with Vista, but not so sick of it that you're ready to face a complete mulligan on the operating system, virtualization might be for you. Add virtualization machine software on the Vista-running PC, create a VM, then install XP into the VM. You'll still need a licensed copy of Windows XP to be legit. Fortunately, unlike Vista, XP's EULA doesn't forbid virtualization. (Only Vista Business and Ultimate, the downgrader's friends, can be legally run in a virtual environment.)
You really have three picks here, including Microsoft's own Virtual PC 2007 (free), SWsoft's Parallels Workstation (US$50) and VMware's VMware Workstation (US$189).
The biggest bonus in going virtual is that if you change your mind -- again -- and decide Vista isn't so bad after all, you can just delete the VM and have your old, or new, machine back.