I've created a RoboHelp2015 project but when I publish it on our Web Server it opens in MS Word.
What flavour of help are you generating in your Single Source Layouts (SSL)? Sounds like you might have made “Printed Documentation” by mistake.
This might also mean that somehow file associations are off. Admittedly it's a very long shot, but still within the realm of possibility.
Where exactly is the server? LAN or Web or Intranet? Does it happen only for you or for everyone?
Yes thank you - it was the file associations that was off. All sorted now.