http://hcidesign.com/memtest/ <-- memtest. Run this and let it run for a long time. It will tell you if there is a problem with your ram. As for checkdisk it is the built in windows file and disk checker. Right click on your OS drive and select properties --> tools --> error checking, and select both boxes. It will ask to reboot to do the test, reboot and you will see a DOS type screen during boot that is where the test is automatically run. Please try checkdisk first. as for memtest some say to let it run for an hour or more but i have always found bad ram within like 15 minutes of it running.