[Daveskater]

On the back of your PC there will either be three, six or none 3.5mm jacks for speakers. They will either be on the motherboard, in which case they will be near the USB sockets or on-board graphics (if it has that). If they are not there then they will be in a line in one of the PCI slots, which means that you have a PCI sound card. You can plug your headphones into the green one, as that is the normal stereo speaker socket.

By the sounds of it you are using built-in sound, so look around all the other connectors up the top half, under the power supply but before the expansion slots, for a green headphone/speaker socket first.

Have you tried plugging them into something else as Gunner said? Pretty much anything that puts out sound will have a headphone socket on, the TV, radio, MP3 player, anything like that. If it is still the same then it will eliminate the sound chipset on your computer and you will most likely have a broken cable inside the insulation on the headphones. Sometimes they break down at the connector end, and if it is down that end then there's not a lot you can do, unfortunately.