uninstalling

I have messenger 6.2, and it used to work perfectly with Windows ME. I have
now got XP and am unablr to log on because of an error and i cannot uninstall
the existing messenger 6.2, Another installation of 6.2 will not work and
neither will messenger 7.0.

Whats wrong and how can i remove 6.2 and install 7.0 in the future?