I'm not sure if this gonna work, but try it:
Go to Tools -> Internet Options -> Security then change it to Medium, if there's a Restore to Defaults available then click it.
If this doesn't work, try to uninstall YM, then delete the Yahoo Messenger folder in the Program Files folder, and run a search for "yahoo" in drive C (make sure u allow looking into subfolders and hidden files), and delete whatever results u may get, then install a fresh copy of YM.
The ultimate solution though is to format drive C and reinstall windows

Hope u wont reach this far!
Take Care