How scary for you!!
You can try the webpage WHOIS which will provide you all the IP information and you can go to
http://www.abuse.net/index.phtml to report the internet abuse. I would also recommend you contact the police, who may have more information about your rights and how to track down the user...
If you can locate the IP address, then you can go back to the user's domain and inform them of the harrassment and ask to have it stopped...save all the emails...don't delete them, in case you need evidence to anyone who requests it...
I wich you the best of luck...if you know who it is (or reasonable suspicion), it might be another reason to contact the police...