if you have nothing important on your PC then you cant beat doing a low level format of your drive(s) followed by shuting down your PC. then reinstalling your OS and software. that said nothing is 100% as they can even infect your firmwares if you let them. try using a live Linux distro for your 'dodgy' browsing/downloading. you dont even need a hdd then and everything you dont transfer to external storage will disappear when you shutdown.