sounds like a profile issue..
reboot your computer..
login as one of the users that works..
rename the folder C:\docs and settings\USERNAME (where username is the account name that is not working)..
Logout and login as the user that doesnt work.. this will re-create the profile..
It should now work.. though you will lose some of your settings.. so copy back favourites/desktop items etc..
If your not happy you can always revert it back the way it was..
The other thought that comes to mind is maybe this 4th account doesnt have enough priviledges to run it.. Ie not a local admin.
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