1) download memtest86+ floppy or iso
2) if you hve the floppy version extract the file and run the setup > if you have the .iso use your cd burning app to burn the .iso to a cd as an image
3) reboot pc to floppy or cd making sure the boot sequence is set to floppy, cdrom, hdd or equivalent
If you see red errors that is bad. If you can complete 5 passes with no red errors you are good to go. However passing memtest doesn't mean your rig is stable, and red errors don;t always point to defective memory.
