Unplug any USB devices. Leave only the monitor, keyboard and mouse plugged in.

Or try this from Tom's Hardware:
You may have simply entered one of the accursed GB 'boot-loops'; typically (though there are other causes) they are caused by a USB device that the MOBO is 'unhappy' with - so before doing this, you may want to unplug all USBs except KB and rodent (never have heard of either causing GB USB to 'spit up'...); while the system is rebooting, depress and hold the power switch until it completely shuts off; once off, either power down at the PSU switch, or unplug it; while it's off the power, momentarily short the RST_CMOS pins; power back up, enter the BIOS, do a "Load Optimized Defaults", and go into the peripherals page and disable "USB storage function" (might be labelled 'USB Legacy Support' or some such - I can't currently access my manual stacks...), do to save & exit - should be good to go...