I purchased a VGA pliteer from Ebay in the hope that i would be able to use a second monitor to extend my windows desktop. I connected it all up but instead of extending the desktop the second monitor just displays a duplicate of what is on my normal screen. My graphics card does support dual monitors becuase the extend my windows desktop option is there but when i try to do it nothing happens.
You don't use a VGA splitter. That just splits the same signal over two cables hence you getting a duplicate image on the second monitor.
Your graphics card should have two outputs. One of these may be a DVI one in which case you will need a DVI convertor if neither of your monitors have a DVI socket. However, some cards just have a TV-out as the second output which will not be suitable for a second monitor but still offers the option of an extended desktop (so that you can use a TV too).
If you have the appropriate outputs, just connect both monitors to a seperate one on your graphics card and you should be able to enable the extended desktop.