I recently bought a refurbished HP/Compaq DC7600 for < $100 for a client of mine. This unit came with Windows XP installed, included a valid (special refurb) COA and a CD with Windows XP and SP 3 slipstreamed on the same disk. That was worth $100 right there!
Powered the unit up and was treated with the Dialog Box, "Can't login, you must Activate first". OK, let's activate. "Cannot connect to activation server". Uh, oh - networking isn't set up, I need to login for that. So, I can't login until I activate, and I can't activate until I login!Zugzwang!
I googled a bit for how other people fixed this one. Got the idea from them to use Safe Mode under Administrator, (NOT Safe Mode with Network). The I used the "Setup Internet Connections" wizard under Control Panel. Rebooted. Bingo! Activation complete.