You'll get one for around £30 in the high street.
Disconnect computer from mains first.
Once you open the case look to see how it's fitted, usually 2 screws or a clip will hold the drive in place. Remove these and it should slide out.
There will be two leads connected in the back of the drive, just swap them over to the new one, they can only fit one way!
Make sure you set the pin in the back to master or slave depending on what the current drive is set at, make it the same.
Put it back in the way you took it out, connect up the mains again and reboot. Windows should detect it and install it.
click here for illustrated help