The -5V and -12V are probably fine with no load, but the +12V line should have at least the minimum required load (whatever that might be). Just use a suitable power resistor (power resistor means it won't fry into a molten lump) on the +12V line. In the absence of info on the PS label, I'd go for loading the +12V line with about 10% of its maximum power/amps. Use V = IR and P = IV to do the algebra to get the right resistance (ohms).
Also, along the lines of Adam's reply, you might have to signal back to the PS (by closing a circuit?) that "this motherboard likes you" - otherwise the PS might cut out.