Here is your problem, the AMD Sempron 140 I believe is a single core CPU that right there leads to the main issue why you would be lagging. The second thing is that you have a GTX 460...why in the world you pair a CPU like the AMD Sempron 140 with a GTX 460 (seriously, it makes no sense for you to do that)?
The problem is that the AMD Sempron 140 is bottle necking the GTX 460, you need to upgrade that CPU to something equivalent to the GTX 460 which could be an Athlon ii X4 or better yet a Phenom ii x4.