I have used Windows XP more than any other OS from Microsoft and in fact I still have a test machine that runs Windows XP.
Back in the days Windows XP was a huge overhaul from Windows 98 and the failed Windows Millennium, however that look has stuck with me for quite sometime.
Also Read: Stunning Windows 7 Wallpapers
If you are using Windows 7 and are nostalgic like me about Windows XP, you can now download a Windows XP theme and use it on Windows 7.
XP Luna is a theme for Windows 7 that will bring the look and feel of Windows XP to Windows 7, definitely worth it if you want the simple looks of XP and the power of Windows 7.
Download XP Luna Theme [via Ask VG]