I signed up here just to say that this happened to me as well, because it's the only forum I've seen where someone else had the same problem as I did (I thought I just had a faulty board).
I can't remember all the specifics of everything I did during my install/update process, but I do know that I didn't do everything exactly the same as Sin0822. Also, the Vcore over-voltage was different (1.66V I think, as opposed to 1.78V). The main common thing seems to be bios version F3, so if there's anyone else on F3, I would advise them to go back to F2 or up to F4A (or later) asap.
Here are some notes about what happened with my rig:
1. At some point while installing/updating Windows and upgrading from bios version F2 -> F3 -> F4A, my PC started to become unstable even when not under load (symptom: instant reboot). I went back into bios and noticed that the Vcore had somehow set itself to 1.66V or thereabouts (I forget if I was on F3 or F4A bios version by this stage, definitely not F2). There's no possible way I could have done this by accident. If it was on the maximum voltage I could theorise an accidental keypress and save without noticing, but the fact that it was about 6 steps down from the maximum meant I'd have to make the same retarded typo 6 times in a row, or had the only narcoleptic moment in my life for half a second while my hand was resting on the left or right arrow key.
2. I didn't change the VTT voltage, or indeed any voltage before this happened. Not exited/saved anyway. I scrolled through some settings to see what the low/high values were after I first installed the hardware, but I most definitely didn't save any of those settings and they were all definitely on default auto settings when I exited bios. I didn't even change any clock frequencies because I was still installing Windows and not yet up to my overclocking phase of my build. If the VTT setting is causing it, it seems that you don't have to save the setting for it to change your Vcore. Later when I saw the Vcore had somehow been set to 1.66V, it was the only voltage that wasn't set to Auto so to me that's proof enough that I didn't accidentally change/save my VTT.
3. I noticed that this board would not often not save/load settings on the first attempt, so even if I did load optimal settings (which I usually do as a matter of habit, but I cannot say for 100% certainty that I did this time) it may not have loaded the settings anyway because I'm not in the habit of saving/loading my settings twice. I never saw it fail to load/save on the second attempt though, but sometimes after this happened I started loading/saving 3 times to make DAMN sure the settings carried properly.
4. The CPU deteriorated quite quickly after the first spontaneous reboot, until I worked out what had happened. I'd say less than 10 minutes of running at 1.66V before I noticed it in bios and reset it back to auto/1.2V. It would start to reboot even in bios for no reason, and was still getting worse even after I'd fixed the vcore. This was at 1.66V, if the voltage was 1.78V like catcherintherye's, it's conceivable that the CPU could fry before it finished its first POST.
5. The first evidence I noticed that the CPU was dying was the Event log in Windows, which had a bunch of administrator log warnings saying that there were problems with the CPU cache, and that the error messages were generated by the CPU itself and therefore very likely to be a hardware fault in the CPU, rather than a dodgy motherboard or incorrect drivers loaded etc.
6. I sent the CPU back to the shop see if Intel would cover it under warranty. This seemed to be an easier option than convincing Gigabyte that they fried my CPU and making them buy me a new one, even though it is almost certainly their fault. All I could do is sign a stat-dec saying that I had not as yet overclocked anything about the board, but then they might question why I updated the bios if the factory version f2 was working fine, or why I bought a board designed for overclocking and then not overclock it. If I sent the board back after I fixed the bios settings, they'd just say there's nothing wrong with it and therefore it couldn't have killed my CPU. Maybe these forum posts might lend weight to any warranty claims from either Gigabyte or Intel.
7. I've only ever had one other CPU fail in my 18+ years of building PC's and overclocking them, it was a Celeron 300A sometime in the mid 90's and died for no apparent reason (my system froze randomly rather than rebooted, CPU was replaced under warranty by Intel). I'm only mentioning this because of the suggestions that catcherintherye might have fried 2 CPU's in a row with accidental static, which to me doesn't seem very likely for someone skilled enough to build a 5GHz watercooled rig