Thanks for all your help but have managed to solve problem. I tried to reinstall Zone Alarm which the PC wouldn't let me do. Made me think. I found a removal tool from CheckPoint; ran it and problem disappeared.

Hadn't really considered software issue as I did exactly the same unistall of ZA, install of new suite with no problems on laptop.

Sorry for wasting your time but your help has been invaluable and very much appreciated.

Thanks to you both,