ok then if you are sure it is correctly fitted to do this click on start button>settings>control panel>system>performance then check that the memory is present and being detected if it is then thats it apart from clicking on virtual memory at the bottom of the box and then check that windows is managing it or you could of you have the hard disk space turn that option off and set aside a portion of the hard disk specifically for virtual memory.
but if the ram isnt being detected it could be due to the fact it has a single strip of 512 ram in it as opposed to 2 256's and your motherboard may only be able to see upto 256 per slot.
windows has a habit of slowing down the longer the it is used have you reinstalled windows since owning the pc this could make a huge difference its self.
if this is not sufficent then please state what motherboard and ram you are using
hope this is more of a help