You should only perform the following tasks if you feel comfortable in handling computer components and I take no responsibility for your safety :-
1. Remove the drive from your old pc, complete with its IDE Ribbon cable.
2. Turn-off the power to your new PC (at the wall socket)
3. Take the relevant side panel off your new pc and examine the layout of your hard drive(s) and associated motherboard sockets. There will probably be 2 motherboard ide sockets. If only 1 of these is in use, then the old hard drive can be plugged into the other one but you will first need to change its jumper setting to SLAVE (There will probably be a diagram showing the jumper settings , on the drive itself) Make sure that the edge of the ribbon cable that has a RED stripe is in line with the red stripe of the existing IDE cable , in the other socket.
4. Connect one of the spare power leads to the drive (like the one that's connected to your new drive. (the orientation of the coloured wires should be the same as on your new drive, but in any case the shape of the connector will show you which way to insert it) Lay the drive on a flat surface, away from any of the other components.
5. Power-up your pc and hopefully, you should see the old drive when you boot-up.
Good luck !