Memory

I downloaded a techrepublic recommended program called MemTest.exe. It said that if my PC could run the program all night my memory was OK (“the proper way to test is overnight – if the machine makes no errors in that period of time then there is nostatistical reason to expect any errors over a longer run.
“.
Well, my PC hit an error within one minute. How good is this program? Is there good RAM and not so good RAM? Is my RAM really no good?