Did you try a System Restore? Probably too late now if you uninstalled/reinstalled YM.
When you uninstall it is very important to not only use the Uninstall feature but to then do a Search from
start/search/yahoo and delete all finds. Then run a registry cleaner and remove all Yahoo entries as well as all green light issues. Here is a good free cleaner:
http://www.hoverdesk.net/freeware.htm
After all that reboot the machine then try to reinstall the messenger.