E-Liam thank you for the help.
I did as you suggested and tried to run the Trend scan but it locked up as the cpu usage reached 100% after less than a minute.
Tried again some time later and this time the PC shut itself down when the scan was only about 10% complete.
Pressed the PowerOn button and nothing there at all. Switched the power off at the wall and came back about 30 minutes later, this time it booted up normally.
There is obviously something seriously wrong with it that is beyond the limited help I can give it. I shall probably take it down to my friendly computer technician soon.
Bigwings, thanks for the suggestion, I have had no complaints about NAV up to now so I will stick with it until it proves defective.
I am sure this problem is not NAV specific because it happens with other programs.