Because I’m always asked this…

Windows XP:

1) Exit Messenger
2) Delete everything in C:\Documents and Settings\Your Name\Contacts\
3) Delete everything in C:\Documents and Settings\Your Name\Local Settings\Application Data\Microsoft\Windows Live Contacts\
4) Restart Messenger

Windows Vista or 7

1) Exit Messenger
2) Delete everything in C:\Users\Your Name\AppData\Local\Microsoft\Windows Live Contacts\
2) Restart Messenger