Depending on the diagnostics program that you are using, there will be several different features available. Right now, two of the most popular are Norton Systemworks and PC Doctor. Both of these programs offer full diagnostics software, good for fixing just about any software related computer program you can imagine.
Once you run the program, you'll be able to select the diagnostic test that you are planning to run. Some offer you a full system scan, which will scan your entire computer and then display any problems that you having. After displaying any problems, the software will normally give you an option to go ahead and fix it. As long as the problem is nothing serious, the diagnostic software can generally fix it.
The more complicated problems, such as hardware failures or hard drive crashes, are a totally different story. They will require a technician who is very experienced, that can rebuild the hardware. Diagnostic software can report hardware problems to you, although most of the time it is unable to fix it.
There are other computer diagnostic software that you run after starting the computer or rebooting, that will check everything from BIOS. These types are great, although the types such as Norton and PC Doctor are by far superior. They offer far more features, including defragment, cache cleaner, and the option to delete your temporary Internet files. These features are great to have, especially if you aren't the technical type.