This is the FM transmitter which I have (although I think that there is a later version available now which gets power from a USB port of the PC). This one uses three AA batteries and is fine as long as you remember to switch it off after use (it has a little red "on" light).
http://www.addonsworld.co.uk/product.php/22043 /0/
There is a cable with a jack plug. I got a two-way jack plug adaptor which fits into the audio out port at the back of the PC. Into this go the transmitter's jack plug and the one for the PC speakers.
The transmitter allows you to set one of four possible frequencies (I use 88.2). On your PC, you choose the radio station you want to listen to, switch on the transmitter and tune your (portable) radio or Hi-Fi elsewhere in the house to the same frequency.
As suggested in the product description, you can also use it to transmit the output of an MP3 player or CD player to a radio receiver (including in your car).