We are down to "Desperate measures"now !

Don't panic!

If uninstalling won't work that way. try this...
Either press the Start button and look up Yahoo messenger on the "All programs" list and select "Uninstall Yahoo Messenger" pressed this?
No joy?

Could be your prob comes from a ""bad install" (we all get them from time to time

)
Then do this .......( I've done the"donkey work" for you

)
click on the Start button find "Search"
when the new window open look down the left of the screen for the items with the green arrows.
Click on the one marked "All files and folders"
(3rd down)
Then when the Window "changes", in the "Top" box called "All or part of the file name" Copy this into it:"C:\Program Files\Yahoo!"
Then press the search button....
You will get the file location "come up"on the right hand side of the Window

Ok. you can now delete the file by right clicking on the item (selecting Delete from the"dropdown" Menu) or opening it? (only 2 files inside and deleting the Messenger file) though I recommend you delete Both OK?
then go to your Recycle bin and empty it(checking that the files have "arrived" there)
Now you should be able to do a "Restart" or "Shut down" with no fears of it not working

Should you now reinstall Yahoo it will go smoothly
