Why don't they just get back to doing Home and Pro? Now they have one called Starter? LOL! Talk about confusing the hell out of people.
XP also has XP Starter Edition.
Here's the thing about XP Starter Edition, it's only available in 3rd world countries. It's cheap, but it is also VERY restrictive. You can't really even get a key in the US, without really trying.
Also, XP has also had more than 2 versions. You have the following:
XP Starter, XP Home, XP Media Center Edition (with multiple versions or MCE, like 2003, 2004, 2005), and XP Professional. There's also various versions of XP Embedded as well.
For Embedded versions, you have XP for Embedded Systems, and XP Embedded. The first one can only boot into an application, and can't really be used as a full OS. For XP Embedded, it's a full OS. MS is now also coming out with XP Embedded Standard 2009, which just has added updates (IE7, .Net 3.5, ect). There's also a version for Point of Service devices, and heck, you might as well consider Window Fundamentals for Legacy PCs.
There's also a Windows XP Tablet PC Edition.
There's also 2 versions of Windows XP that are 64-bit. You have Windows XP 64-bit Edition and Windows XP Professional x64 Edition. The first one is only for Itanium processors and the Second one is for any x32 64-bit processor. However, the Kernel in XP Pro 64-bit is NOT the same one from Windows XP 32-bit. It's the same kernel that Windows Server 2003 x64-bit uses. In fact, XP's version number is 5.1 and Windows XP Pro x64/Windows Server 2003 is 5.2 As you all know, there are 3 service packs for XP, but there's only 2 for Windows Server 2003/XP x64.
People say Vista "complicated" things for the end user (only if you're an idiot though). If anything, it made things simplier.
Vista Home Basic is what XP Home is. Vista Home Premium is what XP Media Center is. XP Tablet edition is gone. That functionality is now added into Home Premium and above. There is likely only to be 1 version of Vista Embedded, unlike XP (which you have to consider had been on the market as the main OS of MS for 6 years). While there is still 64-bit versions of Vista, they now use the same key and the same kernel, which simplifies updating for MS and simplifies things for the end user (plus, they can go to 64-bit anytime they want to).