There are several organizations and utilities that you may need to search to actually track down who this IP address is registered to:
Utilities:
Ping: at DOS prompt, type "ping IP address /a"; The result should give you the computer name (and most importantly the domain name)
Traceroute: at the DOS prompt, type "tracert IP address"; you may get the same results in addition to clues as to what part of the world the IP address resides in
Organizations:
www.arin.net - registry for North and South America, the Caribbean, and Sub-Sahara Africa
www.apnic.net - registry for the Asia Pacific region
www.ripe.net - registry for Europe, Middle East, North Africa, and parts of Asia
If you search a registry for an IP address that is not covered by that registry, you will normally be told which registry does cover it.
Sometimes all you will be told is that a given IP address has been assigned to a given ISP. Other times you will be told exactly which business or even person has registered a given IP address.
You may be directed to search smaller national registries (particularly in the area covered by APNIC). Some of these are:
www.aunic.net - Australia
www.krnic.net - Korea
www.jpnic.net - Japan
Most registries provide procedures for reporting abuse (such as hacking or Spam). If you happen to be located in the same geographic region as where the IP address is hosted, you may have other legal recourses available to you depending on local laws.
Hope this helps.