Well it seems very odd that they want to know your IP, for getting your account back. I have never heard of them ever doing that before, an really find it odd.. maybe you should check for virus and spyware
Most budget ADSL packages do not supply a static IP from what i know of... but you should keep the same one for a few days until your connection drops. So when you visist
http://whatismyip.com it should return your IP. But as you said that did not work, so it seems like you have a dynamic IP address. You could also get your IP address by logging on to your gateway/modem if you can....
Maybe hotmail have a place for you to enter your MAC address, but im not to sure..
You could also try getting contact with your internet service provider, who may be able to tell you what your IP address was on a given date, or maybe you can do that through there control panel (on therewebsite?) maybe in sessions or usage??
But is there no other way to get the account back from msn? have you tried contacting them