Hi ddrabant,
you did not mention how you set up and secured the fresh installed system. Hence this shot in the dark:
The system was re-infected within 24 hours, the same way it was infected before - through open ports or by visiting the same website, using IE. On my internet connection, my system wouldn't survive 2 minutes without a decent firewall and without all recent MS-patches, it would be infected just by visiting websites, if I would visit them. Part of the problem is IE itself, allowing bad guys to exploit its countless security gaps. So if you postponed updating XP with *all* the stuff for some reason, your friend might have done the same thing as she did
before to get in trouble. Never (!) power up the modem without all needed patches.
How old is the XP-Pro CD? Does it contain any service packs?
At least *I* can't load service packs and hotfixes from the MS site without firewall protection which means that I can't connect simply into the internet with a fresh installed XP without SP2 (or at least SP1?).
As always, cross-checking with a "Knoppix" (or similar) "Live-CD" (means booting an alternative operating system from CD) is a good idea to see if this is related to XP, or maybe to a wacky network card / modem / internet connection issue, which is the other idea. Since it is a fresh installation, network setup issues may occur.