Right, i'm stuck! need help.

since I got my Powerbook, i've always used Mac MSN without a problem using a gmail account on MSN, however, i recently changed my username on MSN to a .mac username and it's recently started to cause errors, people can't start chat conversations with me, and if i start a chat with them and they don't reply within 1minute, then it won't deliver the messages....

any thoughts ?