Windows XP added a huge improvement to the user interface and was very popular upon release.
If you are unsure if your system runs Windows XP, check how old it is. Many pre-2007 systems still run this operating system since they can rarely be upgraded as newer versions of Windows are too demanding for these systems that still run this operating system.
Microsoft no longer supports Windows XP and advises users to upgrade to a new PC. Some PC's are upgradeable, but these are few and far between.
Many Windows XP machines are unable to run more current operating systems as many of these are old enough driver support is unlikely to be present and the hardware will create slowdowns.