Be sure that the Dell tech stays around while you start up the PC, and
especially if it asks you to re-activate. Try the on-line re-activation
first. If that works, be happy, Dell has obviously arranged things to make
life easy for you. For example, Dell might have tied activation to a
specific version of the BIOS (inside the moderboard), instead of a weighted
sum of PC components.
But, it is also possible that the re-activation will fail, an 800 number
will appear on the screen, and when you call that number they (Microsoft)
will tell you that re-activation is not possible, because you have an OEM
(original equipment manufacturer) version of XP, which is good on only one
PC. In this context PC = motherboard. If you get any grief from Microsoft,
put the Dell tech on the phone. Do not let the Dell tech go until you have
a working PC, including all re-activation issues solved. I am sure that
Dell has some way to make this happen.