Although the capability to add themes to Windows has been around for some time, Microsoft added a new class to Windows XP called "Visual Styles." Simply stated, these are new widget sets covering everything from the minimize button in title bars to the famous Start button. Windows XP comes with two of these styles: "Windows Classic style" (which looks like Windows 2000) and "Windows XP style." Each visual style can have multiple colors associated with it, hence the ability to make your Windows XP Luna theme look blue, silver, or olive green.
You'd think that programs like WindowBlinds, which have been giving users the opportunity to skin Windows since before XP, would be dead in the water right now. Not true: Microsoft has added a facility to lock out third party skin designers. Right now, the only way to get more "approved" skins is to buy the Windows XP Plus! Pack. The APIs are still around to change skins, so enter Style XP.
Produced by TGT Soft, Style XP is a $19.95 shareware product which effectively bypasses the checksums used by Windows to validate a theme. This allows users to select one of dozens of alternate themes (examples at http://www.themexp.org) to use instead of "Classic" or Luna.
The underlying reason behind Microsoft's third-party lockout, besides the fact that they make money on the Plus! pack, is for reasons of usability. Many skins change the appearance of the familiar minimize, maximize, and close (X) buttons. While stylish, this creates a nightmare when support personnel are supposed to explain how to click the "little button with a horizontal line along the bottom" to minimize a window. I've seen people with ungodly Windows 98 themes installed by their 13-year-old daughters, but those themes still preserve all the appropriate widgets. Alternative visual styles are cool for the power user until the power runs out.
Style XP can be downloaded from http://www.tgtsoft.com. Its companion program, Style Builder, allows third parties to create their own visual styles. Despite its own skinned-like appearance, Style XP is easy to use and takes up very few system resources. I recommend it for any Windows XP user, along with either "Rhodium Edge" or "Soft Body" for a smooth metallic look.