Windows XP: Home vs. Media Center vs. Tablet PC vs. Professional vs. Pro x64
It used to be very simple to choose which operating system you wanted: you wanted an Apple II or an IBM. It's not quite so simple anymore. There's a dizzying array of five different options available for Windows XP alone. And that's granting that you even want Windows XP in the first place. (For now, let's assume you do, because "Windows XP vs. Windows 9x/ME vs. Mac vs. Linux" is a topic I don't feel like handling today.) Let's begin:
Home (included in price on all newer Windows PCs):
Windows XP Home Edition is the most commonly used version of Windows XP simply because it doesn't cost anything extra to the buyer (usually). It delivers most of the basic features you need: Wireless & home networking, compatability with most newer programs, multiple user accounts, and all of the spiffy new GUI enhancements. Home is probably sufficient for the average user, and is much less expensive than other versions.
Media Center (~$50):
WinXP MCE is basically what it sounds like: the standard version used on new "Media Center PCs." These computers have a handful of special features: TV tuners, DVR functionality, even remote controls. Although perhaps useful for someone who plans to use their PC as a home theater, it has no practical application on most notebooks. It is sometimes believed that MCE is a 64-bit OS; I can't find any real evidence of this, but if true, it would make it a worthwhile upgrade if you use an Athlon 64 or Turion 64 processor. Otherwise, it'll just slow the whole thing down to do what cheaper third-party addons can do better.
Tablet PC (included with tablet PCs):
Windows XP Tablet PC Edition is useful only if you have - well, a tablet PC or a convertable, such as the AVERATEC C3500. Installing it on a standard laptop or desktop will only casuse compatability issues and choke performance.
WinXP Pro is the standard version of Windows used in business and government PCs. It features advanced security, better performance (due to fewer graphical frills), and multiple processor support. Professional is a smart upgrade for business professionals, students who need to access their school's network, and hardcore gamers; it isn't really worth the $100 upgrade over XP Home for grandma Ethel to type up her recepies.
Professional x64 (~$200):
Windows XP Professional x64 Edition is the "more power" version of WinXP Pro. This new release is rarely available from the manufacturer on home PCs and personal-use notebooks; however, it features full 64-bit and dual-core processor support, allowing you to get the most out of that shiny new Athlon 64 x2 or Pentium-D. On notebooks, it's slightly more limited in usefulness, but you should look at x64 if you have an Athlon 64 or Turion, a bit of cash to burn, and an insatiable thirst for the best performance possible. As a warning, a lot of hardawre may not have drivers compatable with x64 yet, and the upgrade therefore usually voids your warranty.