Hi

I Spent alot of time tuning my old pc to play games such as GTA4 i had a pentium 4 and an nvidia gforce 7600 gs 256 mb

have you tried going in to nvidia setting and changing the graphics textures to performance rather tan quality, Try exiting any background programs such as Skype msn Facebook media player i tunes etc.

If you have a retailed version of windows you get loads of unwanted programs like such as dells give you dell desktop manager. try exiting them too as they really lag your computer.

also try deleting unwanted had-drive files such as temporary internet files and programs that are not used.

also set windows themes to basic instead of aero as that will take less GPU and processor usage

Hope some of this stuff helps:)