I'm running a Q6600 (2.4ghz) and have overclocked it (as in the screenshot) but once I go over 3ghz, Vista won't boot. It blue screens and restarts. So I put it back under 3ghz, and it told me it needed to find drivers for my wireless network adapter. I unplugged/replugged the adapter and it was working fine again. You can see that my temperature is fine, and it's a problem with Vista as it's a BSOD, not a power out. I'm running 64bit if it helps.
I tried in safe mode, that failed, I tried unplugging my wireless adapter, overclocking then booting, same results.
The reason why your system won't boot after you OC it to 3.0GHz is because your processor can't handle being OC'd that much...you are trying to demand to much from it and thats why windows won't boot... Ocing a processor to much causes more heat to build up and if you run it like that to long you can burn it out.. A good rule of thumb for OCing is to only OC 200-400 MHz, anything beyond that will require a liquid cooling system or other high-performance cooling... P.S. your running a 2.4GHz Intel...you can probably get away with running it 2.8GHz and still be stable...I wouldn't go any higher than that. you must not have much experience with OCing...because that is not a problem with Vista...I bet you if you tried OCing to 3.0GHz on an XP machine you'd get the same outcome...
moved to correct forum as not a windows issue. try posting in this link about your problem. http://forums.afterdawn.com/thread_view.cfm/83263