We just bought a house in an area between two mountains and TV reception is horrendous! We can't really afford cable or satellite so we think we may want to install a decent roof antenna. The house has leftover wiring for cable from the previous owner. My question is whether we can wire the antenna into the existing cable coaxial cable or if we have to install all new cable for the antenna. Thanks for the help! (We are not trying to get cable for free, just to get decent reception).

Are you using a regular anolog antenna?

Here is a good site that will help you select an antenna. AntennaWeb

Coax is Coax. RG-6 for backbone and RG-59 for connects.

If there is more than one set, there may me an amplifier somewhere, but CATV amps generally won't amplify UHF. AMPS can be mast mounted or located elsewhere. UHF has a lot of loss so it makes sense to have gain at the antenna or amplify very soon. Consider a rotator.

Remember what happens in 2009. Digital TV only.

The CATV distribution in the house should be OK.

True 2009 is looming, but you will be able to get your ATSC channels off an antenna with an "true" HDTV (meaning not HDTV ready) or a DTV set top box that can convert down to analog.

The above site is good for determining stations around your area, and direction. You may want to get a UHF/VHF antenna with a pre amp(Channel Master/WineGard/Radio Shack).
There is a cable that connects from Antenna preamp(Mounted on Antenna)to the Power Supply(Plugs in the wall). This One coax sends voltage up coax, and signal down to TV. On the TV you will need to select "Air", not "Cable" in the menu, if you want to get UHF(channel 13 and above), not cable 13 and above. If a Distribution Amp is used, Make sure it handles UHF/VHF, Not cable. This antenna will be great for any FM stations also. Just make sure FM trap is off, unless FM is affecting reception.
As far as the cable, you can use the existing cable. It is on your house, it is yours now.
RG 6 is better than RG 59. It has less loss per foot and handles the higher frequencies better. Just eleminate any splitters between amp and preamp.
Just have all tv's on the output via splitter and or distribution amp.

Thanks sacramento, I meant channel 14 and above, not 13 and above. Good luck.

Here is an interesting resource:

Antennas Direct | Your TV and HDTV Antenna Source