All solved now, it was a faulty on/off switch that was stuck in the ON position and rebooting the machine as soon as it switched off. often the simple things cause the most problems.Tks for your suggestions.
My Del 1720 will not switch off. it just reboots itself everytime. I have to remove the battery and unplug the mains to stop it. Ihave downloaded new BIOS and fitted a new hard disk and new Vista but it still will not switch off. Any ideas?