Some one is calling me and harassing my girls , how can I track down that provert using my computer?
(I only have their tel. number)

And when you find out who it is, what are you going to do, go over and shoot them?
Call the police and make a police report each time they call, they will trace it.

It could be a cell phone, a pre-paid cell phone, a listed number, an unlisted number or more. Let the police handle this.
There are some reverse phone searches but these normally only will show regular home phone services.

Most telephone carries allow you to dial *57 after a harassing call.this records the call details which can then be turned over to the police.

I think the only way to stop it is -- turn the tables on them

Stopping Harassing Phone Calls (http://www.privacyrights.org/fs/fs3-hrs2.htm)