-12 V is used for serial (COM) ports, sound cards, and some graphics cards.
Power consumption is very variable.
A middle-grade PC (Athlon 1000 MHz, GeForce 2, 2 Hard drives) uses about 90 W when idle, and 125 W when busy. A 17 " monitor uses about 90 W.
A simple PC (Celeron 700, 1 slow HD, integrated graphics) uses about 50 W when idle, and about 60 W when busy. A 14" monitor uses about 50 W.
These figures are based on measurements of my PCs using a true energy meter (not by measuring current in the mains cable which is a very inaccurate method).