You could brick your motherboard by flashing it with the wrong bios. You could try to burn out your CPU and GPU using a program to turn the fans off while you stress test them. You could uninstall your anti virus and load your hard disk full of the latest self-replicating worms. And you could go out and try to find one of these to plug into your PC (http://securitywatch.pcmag.com/hacking/326196-an-evil-usb-drive-could-take-over-your-pc-undetectably)