Batman arkham asylum works slow on my pc????

i had 4gb of ram, core2 duo(2.94ghz),windows7 ultimate,graphics card 1 gb(nvidia),,,,no virus in my pc 100%secure,,i also had defragment of my drives...bt than also it works slow... some where it works fast and than again becomes slow....................................… plze help me out gies.........plzeeeeeeeeeee