They are just empty files but have name as follow
WIN51 = Both Win XP and Server 2003 have this file (Doesn’t appear to be required for booting)
WIN51IP = Win XP Pro
WIN51IP.SP1 = Win XP Pro SP1
WIN51IP.SP2 = Win XP Pro SP2
WIN51IC = Win XP Home
WIN51IC.SP1 = Win XP Home SP1
WIN51IC.SP2 = Win XP Home SP2
WIN51IP2 = Windows XP Media Center and Tablet Edition
WIN51IL = Small Business server 2003
WIN51ID = Data Center Server 2003
WIN51IA = Enterprise Server 2003
WIN51IS = Standard Server 2003
WIN51IB = Web Server 2003