First step is to go back to basics and stress test the PC.
Disable/uninstall any programs that can modify BIOS (OC, etc).
Boot directly into BIOS, and load/save default BIOS settings.
Boot into Windows.
If problem has not gone away:
Stress cpu and memory for at least an hour using Prime95 and CPUID's Hardware Monitor to watch temps. Do not let core temps exceed 75C for too long. If you abort due to heat, resolve that problem. If Prime95 reports a hardware error, we'll be looking at psu or memory/memory settings.
Stress GPU again using HWM to monitor gpu temps. Do not let temps exceed 95C for too long. If you abort due to heat, that problem needs to be resolved. If you crash, we have a bad vid card or psu.
If you pass these tests, the odds shift in favor of a software issue somewhere.
Utilities can be dl'd from these links:http://forums.tweaktown.com/gigabyte/30530-latest-overclocking-programs-system-info-benchmarking-stability-tools.html