Ok, I got curious myself and went googling for you. I found this:
http://www.dabrowski.ca/kb/versions.asp - the windows version info is about 2/3 down the page. As you can see, the service pack makes no difference, they are all 5.00.2195. (Many more sites say the same thing - go googling if you dont believe me, but this one laid it out best)
This brought me to remember a key thing about service packs: Micro$$oft have always strongly stuck to the rule that they do not release new code in service packs, just minor modifications to the existing code. Thus, they are not windows builds, they are windows tweaks, and the build number stays the same. The major reason I remembered this was Microsoft have just broken their own rule with the release of the latest XP service pack, as it does introduce new code, for this blocking of the top 20 invalid registration keys in use around the globe.
Hope that helps!