This is the Swiss Knife of computer repair technicians, and it's Free! Check the list below and you'll see that it has all the diagnostic tools that you'll ever need.
Follow the link provided, download it, and burn it on a CD (keep in mind that the download is an image file but most burning softwares will recognize it and directly burn it as a bootable CD).
Restart your computer and let it boot from the CD. Once the CD has loaded on your computer, simply choose the tool that you need (Find a snapshot of the Menu or see the list of tools below).