I've been asked by a few people where they can find the SP1 upgrade for a "Download" as opposed to "express" through the "Windows Updates".
SP1 works on both XP Home and Pro operating systems.
Find SP1 Network install here
. Download from the FIRST box on the right.
Please note that you do NOT need the 64 bit SP1 because I strongly doubt you're running a technical workstation with an Intel Itanium processor.