In general, there is no PAL or NTSC of computers (apart from signals outputted to the TV anyways)...
...i've probably confused you already, but simply put...Yes they are all compatible. Compatibility doesn't vary from country to country, but rather across computers. So you'll just have to take note of things like VGA, DVI, PS/2, USB- your computer's ports, and make sure your PC is compatible with specific hardware.
...well I guess there is a slight compatibility issue with power supplies across countries.
Power supplies in the US are equipped with a two pin power cable (right...?), whereas power supplies in the UK are equipped with a three pin power cable...
I think power cables are the only issue with compatibility when it comes to computers in different countries. All power cables for the monitor and PSU vary from country to country, depending on the electrical standards they have with their power outlets. Oh and not forgetting what scotty said too, about voltage selection .
But the same applies to all electronics you'll encounter in any country.
But it's easy rectifying this issue. You'll just simply need to purchase and use the correct power adaptor for your power outlet. Or buy a socket adaptor.