If its a prebuilt then It might not work Especially if its A dell, Many Prebuilt companies make the Motherboards and Usally the PSU Proprietary, so make sure that you don't need a special Power supply before you Buy.
Edit: Ok its obviously Proprietary, I warn you because the 20 Pin connectors may not be the same on the Power supply, so, It May not work, May fry Power supply, Or May fry Motherboard or both.
And if its An AT power supply, then GL!.