display or no display.....

I have just built my first pc (so be gentle with me....)
Everything is fine and working with XP home installed,
with one exception.
when I boot the pc, I get a display on the monitor. If i
shut down and reboot, the screen remains black. Power off
and reboot - display, power off, reboot - no display.
It is now so predictable, that I know before I turn on
whether i will see anything or not (if there was a
picture last time, there won't be this time, and vice
versa)

This was happening right from the start, before i even
installed the O/S (booting from floppy, formatting drive,
setting BIOS etc.) I thought it might be solved when I
installed the video card drivers but it wasn't. I've also
tried installing the VGA driver that came on the
motherboard disk, but this doesn't help either.

Device manager shows the card as working properly with no
errors or warnings. Power saving settings and BIOS all
seem okay.

Has anyone any ideas? I would have expected it to work
either all the time or not at all if there was a problem.
At the very least it might have been just an intermittent
problem - but every second boot?