If MEMTEST passes, it might be the Hard Drive is damaged or the data on it is corrupted.
(At this point, you should look into using the latest System Restore point. Google for how)
If MEMTEST fails, then your System RAM is defective (at least one stick).
1. Create a DVD or USB stick (need a different PC) for Memtest
2. Boot to Memtest
*If Memtest won't boot you need to either:
a) change the BIOS setting so the DVD or USB is first in the boot order, or
b) Unhook the hard drive (s) temporarily (the two connectors attached to the hard drive)