When I installed the hard drive I thought I'd done everything right. The PC appears to be working ok (albeit very slowly with the clicking noise). I've done some digging and found a website that gives the correct "jumper" settings for installing one hard drive - the block was on 1 and 2 when supplied so I've moved it to 4 and 6 as suggested. Still no joy.
When I switch the PC on now, the 2 CD drives lights come on briefly, the PC whirrs away but I'm not sure if the hard drive spins or not. Nothing comes on the screen at all - within seconds it flicks onto standby mode.
I've forgotten to adjust the BIOS settings before installing the hard drive but I'm becoming increasingly concerned I've done more that that!