Replace the fan. They're not expensive.
If you're running Compaw hardware, that means it's not exactly new.
I run a Compaq MB and the MB requires a chassis cooling fan to be connected directly to the MB for it to boot. When it gets cold out, the fan is slow to start spinning, the motherboard can sense that and doesn't boot, if I leave it on for a minute or 2 then restart it, the fan spins well enough for the PC to boot. I'm either looking for a fan that's better suited to deal with low temps or a way to fool the PC into thinking the fan is still spinning when it's not, and will still allow it to spin when it warms up. Any suggestions?