F12 brings up CPU debugger window?
To get rid of this problem, you can reassign the UserDebuggerKey by changing the registry. There's a Microsoft support page that details the key (HKLMSOFTWAREMicrosoftWindows NTCurrentVersionAeDebug) and what values you can use.
I would think "Pause" would be a better key to use - makes more sense to me, since the only time I've used it ever has been in the boot process and when getting to the System Properties screen (WinKey + Pause). Both of which I'm unlikely to be desperate for when debugging.