The service pak upgrades are free from the Microsoft XP site.
I can't fathom why there would be a difference when you purchase the OS.
Additionally, if you are using the lap top strictly for playing music for your DJ business, I can't see any reason to install any other service pak except service pak 2.
Incidentally, I am running XP with Service PAk 2. I run PCDJ FX. I operate npo other programs whatsoever on my lap top. I never connect to the internet, and all of my music is and has been, ripped directly from CD to an external hard drive (Maxtor 1 TB fire wire)
I have absolutely no operating system problems, hence, I will never up grade the OS any further. There is no reason to. It runs like a charm.