Sometimes boots up ok sometimes not

Hope I didn't overlook this problem being addressed before in my search, here goes:
Sometimes it boots up ok but sometimes I have to press a key on my keyboard to make it boot all the way up, any key. When it doesn't boot up all the way, the place it stops is just before the Windows XP copyright screen. Then I press any key and it finished booting up the rest of the way.
I just put together this system and everything else seems to work ok:

A7N8X-X, XP-2600+, 512 DDR-400MHz PC3200, ATI 9800 Pro Sapphire, XP Pro.

Not sure what else is needed to figure out what's up with this boot situation.

Would appreciate help, thanks.