as long as you have a way of seeing their IP number you can, assuming its not a dynamic IP and they are not using a proxy. but there is no guarantee how precise and reliable the trace is. i imagine law enforcement just go to the ISP with a warrant or something and get the information like that. you can probably find out someones IP by just knowing their email or msn, maybe with some 3rd party software. but its not something i really know that much about so im not that 100% sure.