Most computers that I see in flyers come without monitors and there are very few that do come with them. Why the sudden shift?

Do all Power supply box’s in desktops computers give the motherboard the same amount of Power (Electricity).
My old computer’s power supply box is damage so i want to put my other computer’s power supply box in my old computer to see if the computer is running well. Both computers are Pentium 3 processor.
Do all desktop computer motherboard take the same amount of electricity.
But my other computer’s power supply box is smaller then my old computer’s one?
Do you think it will work?

Could anyone with some experience give me some insight? This computer would be used primarily for basic home computing: word processing, internet, and making music cds. Thanks in advance to those who respond.

I’ve inserted it my computer and get nothing popping up. A friend of mine has a computer with AOL and it automatically works.
Sorry. It’s the mini.