I have a Volvo S40 2005 1.8 ford petrol engine it is slow to respond when accelerating what could be causing this problem

 

Open the hood and look down. It's something in there. ;-)

Seriously, there's no way to give a good answer to your question, since it could be interpreted many different ways (and most of those ways have multiple possible causes). Check the air cleaner condition, hook up an OBD reader to see if it's throwing a code, try to notice if it's otherwise running smooth or if it's idling rough.

 

helps to note the miles and maintenance history. You may need to check the spark plugs to see if any are discolored or oily - which could indicate a weak spark. If the air cleaner is dirty, chances are the intake will be as well upsetting the MAF sensor or the throttle body. Finally you may need to see if you have proper fuel pressure - filter, pump, sensor etc may be in play