Complex sentence: During bootup, some (especially older) PC BIOS versions will wait indefinitely for a user to press a key before proceeding if some basic devices, such as a video card or keyboard, are not installed or connected, effectively halting an unattended system.
Split into simpler sentences: During bootup, some (especially older) PC BIOS versions will wait indefinitely for a user to press a key before proceeding. If some basic device, such as a video card or keyboard, are not installed or connected, this could effectively halt an unattended system.