Not trying to be insulting but you have of course checked that there is water in the engine and rad.
Has the thermostat been changed recently?
Do the hoses warm up as the engine warms up?
How long from start up to overheat?
What tells you that it's overheating?
Simplest answer , thermostat is stuck.
Unpleasant answer , head gaskets blown and the waters all going out of the exhaust.
More info please.
Dave the wrench., January 2009
