MemTest is a free RAM (memory) reliability tester for Windows OS. It evaluates the ability of a computer’s RAM to store and retrieve data accurately. Unlike other memory checking software, MemTest is designed to find all types of memory errors including intermittent problems. Therefore, it needs to be run for several hours to truly evaluate your RAM.

MemTest tests the ability of your RAM to store many different unique bit patterns, and the ability to correctly hold those values over long periods of time. MemTest catches both immediate errors and long term errors. The longer you run the program, the better the test against long term errors.

MemTest manual- http://hcidesign.com/memtest/manual.html
Homepage- http://hcidesign.com/memtest/