I believe you
need to have a password setup on your Windows account in order for Remote Desktop to work; if you don't have a password setup I don't think it will even allow you to connect, assuming you have properly forwarded the ports in your router and have enabled Remote Desktop through the Control Panel.
You can add a password to your account in Control Panel->Users, somewhere around there.
Quick and dirty ways to find the two IP addresses you'll need:
Local IP: (The one assigned by your router, also the one you'll need to forward the port to in your router admin)
Start->Run->cmd->ipconfig. The numbers beside "IP Address" is the one you are looking for; the numbers next to "Default Gateway" would be the numbers you'd type into Internet Explorer or Firefox to access your router's admin settings.
It will probably look like 192.168.1.x, where x could be any number but usually starts at 100.
ISP IP Address: If you surf to
http://www.whatismyip.com it will spit out your external IP address, which you would use in the Remote Desktop to connect to your computer from an external one.