There is quite a good free on-line diagnosis site called PC Pitstop that can be found
here.
Taken from the site:
Our free automated tests will get your PC running faster, make it more stable, and identify security problems. The diagnostic tests don't change any settings on your system. The results will show you the details of your PC's operating status, and the advice will help both experts and novices to find and fix system problems.
It also allows you to compare your system against similar systems that have been tested on the site.
I have found it quite useful.