if you have windows xp check that you dont have windows messenger 4.7 and msn messenger 6 both auto loading at start up and trying to sign you in maybe.
windows messenger 4.7 is installed with xp and if you have then downloaded and installed msn messenger 6 maybe both are set to start up with windows and are conflicting.
you can keep both on your computer ok and i would not bother with the hastle of removing windows messenger 4.7 from xp but just disable it at start up if you now use msn messenger 6 or vice versa.
click the little icon in your task bar for each one or start them up from all programs menu (im guessing you have both running and they should show in the task bar as two little 'people' choose the one you dont want to use then for windows messenger click 'tools/options/preferences' then un-check 'run windows messenger when windows starts' or for msn messenger click 'tools/options/general' and untick 'automatically run messenger when i log in to windows'
as above you can have both running at start up but can only sign in to one at a time. either that or sombody else is using your log in.