PCI modem causes reboot with new 64 bit board.

Just completed my first Athlon 64 /Gigabyte computer for a local person.
Very nice; faster than mine. However there is a problem getting a modem to
work, which, of course I did not connect to an ISP before I let him have the
machine as I did not know his password, etc. It gets to the point of
checking username and password but just at the point where the icon should
zoom down to the taskbar the machine reboots. I have tried two modems with
XP default drivers and the drivers on the CD, removed various programs from
the start-up sequence using Absolute Uninstaller, all to no avail. I have no
idea what is going on but would like to solve it. Any experience/advise
please?