So I've been having this problem with MSN Messenger or Windows Live Messenger (WLM) as it's now known - just refused to sign in! A commonly accepted solution is to delete all previous settings, either manually or by downloading a file and running it (http://messenger.jonathankay.com/downloads/remove_msgrsettings.reg)
Well that didn't work for me either - the error I was getting was that I appeared to have an invalid IP - found that quite weird as I'd refreshed my IP several times and no other application was giving me trouble - indeed I could sign into Web Messenger and eBuddy.
Then I just realised that my winhttp settings were still set to a proxy and voila it worked!
Here's how you go about resetting your winhttp proxy:
1. Start --> Run --> cmd
2. Type "netsh" sans quotes
3. Type "winhttp" sans quotes - this is a nested command (more info in a previous post)
4. Type "reset proxy" sans quotes
5. Close the command prompt window
Voila! It works! =)