How can i make this old computer run gta iv?

You need a new computer full stop. There's really nothing here besides maybe the hard drive that won't have to be replaced. You'd need a new CPU, which will require a new mobo since it sounds like you have a Socket 478 P4, and there is a Socket 478 CPU that's fast enough for GTA IV. You'll need new RAM, because the old DDR1 RAM isn't compatible with anything made in the last 8 years or so. And you won't be using the AGP/PCI video card either because newer boards don't have AGP ports and even integrated video is faster than that S3 crap. So you can see where it really wouldn't make much sense dumping hundreds of dollars into your ancient computer.