yeah usually the prebuit bs systems have locked out bios setting where you cannot change any good that will result in faster speed.
and yes any overclocking will result in void warranty. overclocking on stock cooler is never a good idea too. the processor will get really hot and you will have to monitor the temperatures.
someone else should suggest you some software that will allow for you to overclock the cpu in windows, rather than bios. amd platform has the AOD (overdrive). i'm not sure what intel has.