if i want to buy a new case, can i just transfer all my old componants from the old case to the new case or do i need to buy new componants, like a motherboard?
As long as the new case is big enough for the motherboard, you will be fine. The only thing to watch out if you have a full-sized ATX board, as it will not fit in a mini case.
Vern is right in that, back in the day, the different companies used proprietary parts to keep people from using anything but their stuff. As far as I know, all of them have left that practice. If the computer is less than five years old, it should be fine.