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. So it's worth installing SP1 (not SP1a!) alongside SP3.