In protected mode, first 32 interrupts are reserved for CPU exceptions. Every time Windows shows a "Blue screen of death", such an exception has happened. For instance, 0E is General Protection Fault and 00 Division by Zero.
For compatibility reasons, Intel compatible CPUs boot with real mode. A real mode task can be run under protected mode in Virtual 8086 mode (however v86 is a mode only in name).
In addition, even when protected mode in enabled, memory above 1MB is not accessible due to memory wrap-around designed to mimic 80286s. This is fixed by enabling the A20 Line