I would download a program called Memtest86+, and run it for your RAM one stick at a time for several hours. If any errors come back, then you have a bad stick of RAM. If that is good, then I would take your overclock down to stock settings, see if that fixes the problem. If so, then you know its instability due to overclocking.
If all is still well, move on to the GPU and also run a diagnostic on the HDD. Eventually, you'll narrow it down. Or, it could just be windows being windows.