TekSIP is a SIP Registrar-type proxy without status, designed specifically for Windows systems. For each SIP point, TekSIP tells you its corresponding IP address, the port, the date and time.
TekSIP is a completely free tool that only needs half a megabyte of space. It works with both Windows XP and Vista. Its creators allow you to use it freely in both domestic and professional situations. The only thing they ask is to provide them with feedback on the incidences that ocurr while you use it.