XP SP2 seems to be a big change internally. Many programs, such as Firefox and Opera, started requiring XP SP2 at some point, cutting off support for SP0 and SP1. According to a YouTube video essay, SP2 was focused on security. SP2 replaced WMP8 with WMP9, removing all WMP8 goodies such as skins and Like Humans Do.
Another change during the XP era, coincidentally, is Sun suing Microsoft, which caused the removal of the Microsoft Java Virtual Machine, which was added in SP1 but removed in SP1a. However, it can be installed back by installing build 3805 and then build 3810 (last version according to Wikipedia). Installing build 3810 directly doesn't work.
So it's worth installing SP1 (and preferably not SP1a) alongside SP3.
See also: BetaWiki
The main interesting editions are Media Center Edition (MCE) and Tablet PC Edition (TPE). MCE includes Windows Media Center, and TPE includes Tablet PC tools (also applicable to touchscreen) and the game InkBall.
MCE had 3 releases:
TPE had 2 releases:
Both edition share the following unique traits. They have 2 CDs. CD1 has identical content with Professional SP1/SP1a, and CD2 includes both MCE and TPE components, .NET Framework 1.1, and nothing else. The product key you enter during setup determines whether any component gets installed. If you use a MCE key, only the MCE component gets installed. If you use a TPE key, only the TPE component gets installed. If you use a Professional key, neither component gets installed. Because of this, it should be possible to use a Professional SP1/SP1a CD as CD1, and it should also be possible to combine both discs into one by simply copying over the files, although it would exceed the size of a CD.
Note that, as emphasized above, TPE 2002 CD1 is SP1a while MCE 2002 is SP1, with MSJVM build 3805.
| MCE2002 | WMP8 |
| MCE2004 | WMP9 |
| MCE2005 | WMP10 |
| TPE2002 | WMP8 |
| TPE2005 | WMP9 |
MCE 2002 only supports English. MCE 2004 additionally supports de, fr, ja, ko. MCE 2005 supports the 5 aforementioned languages plus it, nl, zh-cn. MCE 2005 also has separately installable localization packs for cs, da, es, fi, no, pl, pt-pt, ru, sv, tr, zh-hk, zh-tw, presumably installed on top of the English version just like with normal XP.
English MCE 2004 is only shipped as an update CD to be installed on top of MCE 2002, while de/fr/ja/ko are shipped as 3 CDs: the first two are similar to MCE 2002, and CD3 is similar to the English MCE 2004 CD. If CD3 is not installed, the resulting system has WMP8 instead of WMP9, but still has Media Center 2004, which is not fully functional in this state, as playing any media requires WMP9. Only the English MCE 2004 CD contains Q810936.exe that updates Media Center to 2004.
Only MCE 2005 has the Royale theme. MCE 2002 and 2004, as well as TPE 2002 and 2005, use the Luna theme.
There's a "Windows XP 2005 Edition with Service Pack 2" that integrates both MCE and TPE components in one install. It seems to use the product key mechanisms from TPE 2005, while adding MCE 2005 components via a series of rundll32 calls.
Wikipedia claims that "Service Pack 2 for Windows XP includes Tablet PC Edition 2005" which is not correct. Non-Tablet PC Editions of XP SP2 and SP3 still don't include Tablet PC components. While the section about TPE on Wikipedia doesn't cite any sources, a Microsoft news release cited by BetaWiki is probably where that statement comes from: "Included as part of Microsoft® Windows XP Service Pack 2 (Windows XP SP2) with Advanced Security Technologies, this free upgrade..." I think what actually happens is, when you install SP2 on a TPE2002 system, the Tablet PC components are also updated to the 2005 version, so you don't need to re-install the entire OS just to get those updated components.