I am trying to add a computer to an existing home network. (XP Pro windows home network) There is an unused tv cable that goes into the room where I want to add the computer. It would be handy if there was some sort of adapter I could use to change the cabling from 10/100 into cable and then back into 10/100. Anyone know of a product that reliably does this?

Yes I know I can go wireless but I have had bad luck with wireless in my area due to RFI. (I think) SSo any ideas?This isn't a solution using the TV cable but take a look at Homeplug.
http://www.pcworld.com/article/id,85003-page,1/article.html (it's a slightly older article but it describes the system pretty well.)
With it you can get network speeds up to 200mbps depending on the equipment you buy (theoretically at least).
Why don't you TRY with simple plain ethernet UTP cable? It's cheap, no other equipment required to buy (if you already have the network cards), just respect length limitation (100 meters maximum cable length between switch and computer) and all should work just fine.

TV cable... You like TROUBLE, don't you? Once there was USED the BNC cable, and that could be replaced with tv cable (in certain circumstances). I haven't seen a network card with BNC connectors for 10 years... Yes, you could try to buy modems (MOdulator - DEModulator, transforms from some sort of signal in another signal), but I guarantee you that will cost more than BUYING a UTP cable.