Depends on the load on the other rails and how good the psu is. Not all the rails are directly regulated, but depend on the load on, say, the 5v or 3.3 volt for a reference. If either of these are heavily loaded, the psu outputs more energy to keep the voltage constant on these two rails, while the others will rise. Good quality psu's make a better job of regulating the various rails, but there's always a tolerance. The -12v may not even be used, other than for things like serial ports, which many machines don't have now.
As for the fault, give the machine a good dust out, remove and reseat cpu, memory and any accessory cards in the slots, power connectors to the motherboard and drives. If you don't have an anti static strap, ground hand to machine case *before* removing components. If this doesn't fix it, most likely a hardware fault on the motherboard or one of the accessory cards...
Regards,
Chris