Help… I cant install Vista on my machine running vista

Hello,
Im running a intel quad core 2.4 ghz machine on a Asus p5kr motherboard with 4 gig of ram. I have a gigabyte 128 mb video card and three SATA drives. The machine is running Vista home premium and ran it perfectly for months. recently it got infected with a virus. nothing crippling, but now the box is showing early signs of going downhill. So I want to reinstall the OS. But everytime I try to do it from the boot disk. it loads up to the first install window and then just stops. Im left with a snazzy Vista background window and a movable mouse… but nothing else. And yes I have tried installing it with just one ram module in… No joy….. Please anyone who knows anything about this help if you can… I am at my whits end. Many thanks….