If you want to reuse a hard disk that you've salvaged from an old PC, the easiest solution is to fit it into a disk caddy. A caddy is a specially designed box that converts an internal hard disk to an external one that can connect to your PC using USB2, firewire or even external SATA. You can buy a USB2 caddy for about Rs. 800.

These external casing has a standard USB2 connector, and it also includes a fast external SATA (eSATA) connector. Fitting a disk inside a caddy is simple. After opening it up, you'll see that every caddy contains the same two cables that you'd use to fit a disk inside a PC: one power connector and one interface connector.
It's vital to buy one that suits your disk, as both PATA and SATA versions are available. After connecting the disk to the power and interface cables you can close the caddy and connect it to your computer.