Has anyone seen this?

I turn my PC on, the motherboard and bios messages come up as expected, and
just as the "starting windows" bar at the bottom of the screen is supposed
to begin, I get a blank screen. Nothing.

If I turn the machine off and on, or just hit the reset button, I've got
about a 50% chance of the OS loading. If the startup bar goes across the
screen, the OS loads fine and I can stay on for days with no problem if I
want, no unexpected reboots or anything like that. It's as if the hard
drive just doesn't feel like loading Win2k at times.

If you have any ideas on what might be going on there and where to check,
I'd appreciate it. I've run chkdisk on the drive, and had no problems
reported, so maybe there's some short on the motherboard, but then why
wouldn't my machine be able to stay up for as long as it does whenever I
*do* get the startup bar to appear? Strange one, all right.