Some are better than others, but their diagnostic abilities do not always hold up to what they claim. Keep in mind while running these diagnostic checks that they are only a tool operating on an already sick computer. With these basic limitations always in mind, diagnostics can sometimes be useful. Installing a replacement hard disc would be indicated here, followed by running the diagnostic on the new drive as a final confirmation. Error messages can be very misleading.
Some error messages are useful, while others consistently report erroneous messages.
If you get an error message check out all available resources to track down the meaning of the error message. Don't forget to look in the manual and any other sheets of paper you received with the hardware or software causing the problem. Last but not least go to one of the search engines and enter the error message. You might be surprised how many other people have had the same problem and got their answer online. Each and every piece of software or hardware you add to your system adds more and more files.