Windows XP, also known as Windows NT 5.1.
The first version of Windows NT aimed at home users.
Windows XP had the longest run of any consumer version of Windows. It lasted from 2001 to 2007 before the next consumer version of Windows was released (Vista).
At this time, Windows XP is the OS of choice for most people. It has the stability and functionality of Windows 2000, but with an extra bit of polish to make it look and run better.
It isn't as pretty as Windows Vista, but it runs so much better.
Signed themes from Microsoft that do NOT require any UXTheme patching or extra software.
Just extract these and they work on Windows XP.
Unofficial Windows XP patches are available to allow unsigned themes, and to remove the 10 half-open connection limit introduced with SP2.