You can drain and remove a radiator in isolation without having to drain the whole system.
Just turn off inlet and outlet valves and undo the nuts to the radiator.
I use a paint tray, jug and bucket to collect the water.
It speeds things up a bit if you undo the bleed screw a bit after the water begins to trickle out.
When you do lift it from the wall close the bleed valve and cover the two water holes with clingfilm to keep any remaining sludge in. Best carried upside down for the same reason.
With one rad removed all the other radiators will continue to operate normally.
Peccavi, January 2009