sorry to but in on this conversation -
the length of time the diag takes to run will depend on the amount of ram you have and the size of the hard-drive.
I would run some virus scans in safe mode to rule out any virus or root kit - use malwarebytes scanner but dont enable the trial version - just use the free and do a complete scan, that will take awhile to depending on the size of the hdd. Also make sure you run the scan in safe mode - DOnt forget to take MattyB's suggestions for a system restore and trying one memory stick at a time - DId you add more memory recently? Check to make sure the sticks are seated correctly - and make sure they are the same speed - it kinda sounds like a stick has gone bad or is not seated correctly...just trying to give you ideas and some more guidance - take it with a grain of salt and good luck - will check tomorrow for your results...