Although Dell and some other companies offered past buyers the chance to "downgrade" their new computer to Windows XP from the unpopular Windows Vista, it's unlikely those same offers will be made with Windows 7, which is a very solid operating system.
Scenario 2: You have a computer with Windows XP, and are not sure whether to upgrade to Windows 7 or not. This situation is less cut and dried. Windows XP originally came out in 2001, which is the Stone Age in computer years. There are a number of new programs which Windows XP doesn't handle well, or at all. On the other hand, you know Windows XP, and if you've had it this long, chances are you like it.
If it does what you need it to do, and don't mind being behind a bit technologically, you can stay with Windows XP. Be aware, however, that you'll probably be paying for support, and eventually, Microsoft will stop supporting and providing fixes for Windows XP.
If your hardware can handle Windows 7 (more on that in a moment), you could install it on your hard drive and replace Windows XP. There is no "in-place upgrade" from Windows XP to Windows 7; with an "in-place" upgrade, the new operating system is installed over the old one, keeping all your programs and data intact. To get Windows 7, you will have to do a "clean install," meaning erasing your hard drive, installing Windows 7, and the reinstalling all the information, including programs and data, that you backed up before wiping your hard drive.
To find out if your computer can run Windows 7, download Microsoft's Upgrade Advisor and run it on your system. If it says you can run Windows 7, go for it.
Scenario 3: You have a computer with Windows Vista, and don't know whether or not to upgrade. This is the stickiest situation of all. Keep in mind that Windows 7 is based on Windows Vista; it's essentially the next generation of that operating system, with many user-friendly tweaks. It's like buying the 2010 Ford Mustang, or trying to save a little money and getting the 2009 version – it's basically the same engine as last year's model, but the look and feel has been enhanced and refined.
Windows 7 has some nice upgrades over Windows Vista, generally snappier performance, and fewer annoyances like the endless pop-up windows that ask your permission to do almost anything. It's cut out some of Windows Vista's fat, and replaced it with a cleaner, better look.
If your computer can run Windows Vista, it's almost surely able to run Windows 7, since the hardware requirements are very similar (although it still makes sense to run the Upgrade Advisor, just to be safe). Windows Vista also offers an "in-place upgrade" path, allowing you to install the new operating system without erasing your hard drive and starting over again from ground zero (although many experts still think doing a clean install is the best way to move to a new operating system, because fewer issues are encountered that way.)
If you feel like your computer is pokey with Windows Vista, or there are a few "must-have" new features you simply can't live without, it makes sense to switch to Windows 7, either via the in-place upgrade or a clean install. If you've tamed Windows Vista, however, have it running smoothly and personalized it for your needs, you don't need Windows 7. Remember that they're first cousins – not complete strangers, the way Windows XP and Windows 7 are.