i just installed a new mobo (Elitegroup PF21 Extreme) and i got the
BIOS set up to where it boots from the HDD and all, but when i get to
the WinXP boot screen (black, with the little bar in the middle that
looks like a progress bar) it sits for a few seconds and restarts. i
don't have a WinXP CD because i bought an HP packaged computer
originally, planning to upgrade. upgrading has gone fine until now
with the new mobo. it boots to that screen alright, but it keeps
restarting. what to do?

Yep, that's normal. The OS has all of the virtual device drivers for
the old board, and it is confused. (It's amazing, the things they
don't tell you!) The system may settle down after 10-15 reboots, and
that will not hurt your system, so let it run for a while before
deciding that you must reinstall / repair the OS.

Since you do not have an XP cd and swapped the board, the HP recovery
CD may refuse to allow you to reinstall. There should be a tech paper
on the HP site on how to get around this issue, which usually involves
simply adding a command line parameter to the install command. An HP
tech support rep can walk you through it.

FWIW, I recommend a complete re-install of XP, which will fix the
problem and is probably less of a hassle than the repair install
anyway.

Your other option is to drop $95 on a copy of XP SP2, and perform a
new install.