Windows XP (NT 5.1)

With Windows XP (allegedly the "XP" stands for "eXPerience"), Microsoft has given the UI a complete overhaul. And holy crow does it ever look pretentious.
Windows XP is now the only version of Windows Microsoft is selling, and that marks the end of DOS-based Windows. Microsoft finally stepped everyone up to NT, wow it took them almost ten years!

Under the hood, Windows XP is nothing more than Windows NT 5.1, which suggests it has very little difference from the core of Windows 2000 (NT 5.0). There are both Home Edition and Professional versions, these screenshots are of the Home Edition.

Because of the color gradients used in XP, some of these screenshots might look a little weird depending on your video settings.

The Windows XP boot splash. Note the green loader bar and "Home Edition" text; Professional changes the text and has a blue loader bar. Later service packs replaced the boot logo with the same blue bar and no text for all versions.

Windows XP setup has kind of changed; there is still a text mode component, but most of the second half of setup is done in the GUI environment now, similar to how home editions of Windows have worked in the past.

The Windows XP "out of box experience" first run wizard has changed significantly as well. It now plays extremely cheesy background music (where have I heard that before? :P) and has a stupid animated question mark icon bounce around and make irritating noises.

Signing in is now done through this fancy looking screen, with huge buttons as well as the user's names. Note that if you have a single-user setup, you will be logged in automatically (as shown on the left).
It is still possible to configure Windows to use the old-style login window, and it can be accessed directly by pressing Ctrl + Alt + Delete.

The first new thing Microsoft introduces with Windows XP is the "Windows Genuine Advantage" technology, aka Windows Activation. Or the "Genuine Disadvantage" if you want to be accurate about it...

The goal here is to prevent software piracy. Good luck with that!
You have two options. The first is to use a LAN/dial-up connection and type in a 25-character product key, just like you would do during setup. Or, if you either don't have a connection to the internet or XP doesn't have a driver for your NIC, you must phone into Microsoft and give them a LOOONG (48-character) installation ID, and they will give you an equally-as-long one back for you to type in.

You had better hope that you can access the internet while installing XP, or that you are not dyslexic in any way. And you had better hope that Apple or the DOJ hasn't put Microsoft out of business, because if you can't activate then you are SOL (unless you crack it, there are a number of those out there). I also expect that when XP runs end of support then activation will no longer be possible either, in an effort to force people to run out and give Microsoft more money.

UPDATE: As expected...although I guess that activating over the phone may still be possible if the format of the install ID is still compatible. I haven't activated XP over the phone in eons, but Vista does still work over the phone (and it's also now required for Vista for the same reasons as XP).

FURTHER UPDATE: It is indeed still possible to activate XP over the phone. You could also install something called Legacy Update, which is intended to allow older Windows installs to update themselves but apparently also includes the necesary fixes to repair online activation in XP.

If you don't activate within the 30 days Microsoft gives you, it will refuse to log on. Clicking yes brings you back to the activation screen, and that is all you may do.
If your hardware changes "significantly", Windows will re-prompt you to activate.

At best this whole WGA crap has prevented only a few nitwits from installing unlicensed copies. It has also presented an annoyance to customers with legal copies when they go and try to reload their machine after a crash.