Good hardware diagnostic tool

I am looking for a good hardware diagnostic tool. Actually I?m also wondering if I really even need one. What makes me look is the time I seam to waste troubleshooting legacy machines at customers sites. I?m wondering if I had something that could look inside the box and analyze the hardware to at least eliminate that so I could go on and work on the OS. Has anyone any ideas on a good tool to analyze and troubleshoot hardware on-site? I have looked at this one here http://www.micro2000.com/microscope.htm but have not bitten off on it yet due to the cost. Has anyone bought this one?