So I think you should somewhere written "Includes Service pack 3" see the picture:
http://www.amazon.co.uk/WINDOWS-XP-Pro-1PK-SP3/dp/B0018RCAD4/ref=pd_sxp_f_i/277-8948393-6826463
(click on the picture to enlarge)
At root of the CD Windows XP Professional with SP3 - I see the files:
WIN51
WIN51IP
WIN51IP.SP2
WIN51IP.SP3
At root of the CD Windows XP Professional with SP2 only - I see the files:
WIN51
WIN51IP
WIN51IP.SP2
LC