Prime95 is a software which searches for very large prime numbers. It has also gained a reputation for getting errors on computers which are even slightly unstable. If there is any problem with your CPU, your RAM, or the parts of your motherboard which are the interface between your CPU and RAM, then Prime95 has a tendency to fail. This is a very useful characteristic if your computer is flakey and you're not sure which component is causing the problem. If your computer can run Prime95 for 24 hours without any errors, then you can usually assume that your CPU and the parts of the motherboard which interface with the CPU and RAM, are stable and working well. The software is free and you can download it from here.

Prime95 is a software designed to be used to find Mersenne Prime numbers.

Mersenne numbers can be proved composite (not prime) by either finding a factor or by running a Lucas-Lehmer primality test.

Prime numbers have long fascinated amateur and professional mathematicians. An integer outstandinger than one is called a prime number if its only divisors are one and itself. The first prime numbers are 2, 3, 5, 7, 11, etc. For example, the number 10 is not prime because it is divisible by 2 and 5.

A Mersenne prime is a prime of the form 2P-1. The first Mersenne primes are 3, 7, 31, 127, etc. There are only 40 known Mersenne primes.

There are two ways to use this program. The automatic way uses a central server, which we call the PrimeNet server, to get work to do and report your results. Anyone with Internet access, including AOL, CompuServe, and dial-up ISP users should use this method. You do not need a permanent connection to the Internet.

The second method is the manual method. It requires a little more work and monitoring. Recommended for computers with no Internet access or with some kind of firewall problem that prevents the automatic method from working.

Torture test your CPU with Prime95
Installation And Running Tests
Prime95 is a distributed software which spreads the work among large numbers of computers. When you run it for the first time, click the "Just Stress Testing" button when asked. But if you would like to contribute some spare CPU cycles to help find prime numbers, by all means press the "Join GIMPS" button.

To test your computer, run Prime95 and click the "Options" menu and then select "Torture Test".
Then select a test and click "OK". In this case it's "In-place large FFTs" which is the best test to run for single core CPUs. Run the test for a few hours. If you want to be thorough then let it run for 24 hours. To stop the test, click the "Test" menu and then click "Stop".

The example above shows a successful run. Prime95 checks its calculations against known results so if something goes wrong it will stop automatically. It's also been known on occasion to crash unstable machines.

If it crashes or finishes with an error as shown above, then you've got a hardware problem to deal with.

The license of this software is Freeware, you can free download and free use this diagnostic software.