MS Outlook on NT4.0 WKS

On one of my WINNT 4.0 workstation is used by number of people at differnt times. Now MS outlook is not opening for some users. Whenever they click on outlook icon they get ‘MS outlook cannot open’ message. As a local and LAN Administrator I can start outlook. One of the user is also able to use outlook on the same workstation. The other users can access their mailbox through outlook on other workstations.
Any Ideas????