Six hours ago Ina Fried wrote that Windows XP is a hot item at Amazon.com. The full version of XP Home was number 15 on the software hit parade and the full version of XP Professional was number 21. Amazon updates the list hourly. As I write this, XP Professional is up to number 14, though XP Home slipped down to 16.
There are many ways to slice and dice Windows XP, but I'm going to focus on three "families" - full (expensive), upgrade and OEM (cheap).
The two best selling versions at Amazon are from the "full" family. Full versions of XP can be installed on a virgin computer, or more likely, a virgin virtual machine. My guess is that Mac users are gobbling up the full editions of XP to run in virtual machines alongside OS X. I say this because Mac OS X Leopard is number 7 on the list, VMware Fusion is number 5 and Parallels Desktop is number 17. Fusion and Parallels both provide virtual machines for OS X.
I also think this because the more expensive full versions of XP are outselling the cheaper upgrade versions.
The upgrade version of XP is what most people buying a shrink-wrapped copy of the operating system purchased over the last seven years. Before an upgrade edition of XP installs, it has to find either an older copy of Windows already on the computer, or you have to provide it with a CD of an earlier copy of Windows. The description of the upgrade edition of XP at Amazon.com is wrong. It says "Upgrade only; previous version of XP required." You need a previous version of Windows, not a previous version of XP. For example, upgrade versions of XP will install fine when presented with a copy of Windows 2000.
Both the upgrade and the full versions of XP share a common trait, they are retail editions. As such, tech support is provided by Microsoft and you can call them on the phone for help. I forget the exact rules but the first couple or so calls are free. At least until April 2009 when Microsoft will no longer offer free tech support for retail copies of XP.
Finally, there are OEM copies of Windows XP, sold by retailers such as NewEgg (which also sells the full and upgrade editions). These are the cheapest way to go, but they include no tech support at all. The intended audience for OEM copies are small companies that build computers. When you buy a computer with Windows XP pre-installed from such a company (often called a "system builder") they provide tech support for Windows, not Microsoft.
Another difference is that the retail copies of Windows XP can be installed on one computer at a time. If the computer dies, you can move that copy to a different machine. Not so with the OEM copies. They are married to the computer they are first installed on. If it dies, so too does your license to run that copy of Windows.
For those of us that prefer XP over Vista, an important difference between the OEM and retail editions is that Microsoft still offers the OEM editions. The retail versions are being from stock by retailers. When the stock runs out that's it.
Who can buy OEM copies of Windows XP? According to this June 24th article at PC Magazine, anyone willing to live by the OEM rules.
Of course, buying a shrink-wrapped copy of XP is only one many ways to still get your hands on a copy.
Windows XP will be supported by Microsoft until 2014, so an investment in a shrink-wrapped copy won't sour.