Do i need to count all the cables connected to my psu and measure the dimensions of the prebuilt power supply before getting a new power supply?
Most PSUs are of the industry standard "ATX form factor". Unless it's a Dell, then you might have an issue. WHat specific computer do you have?
Anyways, this standard means that most PSUs should just fit right in, no hassles.
My prebuilt system is an Acer aspire tc780.
Grab an ATX power supply. Just familiarize yourself with the cables - The 24-pin Mobo, the 8-pin CPU, the PCIe wires.