Did u follow Vhockey86's suggestion? If you look in the advanced display properties, dependant on your video card, you should find the display tab. From here you can turn on the tv display (ati cards). You may first have to click on the second blue square (representing ur external tv monitor) in display properties --> settings first. Make sure exerything is plugged in first and turned on. On some new tvs the svideo must be selected over the scart imput for the 3rd port.
Don't worry about the cable - as long as it plugs in at both ends and in good condition it should be fine.
Using the external tv display is different with each card, so play around like Vhockey86 suggested till u find what works best.
Good luck
PS welcome to Techimo!