Is it possible to hook up a Mac to a PC via USB, and have direct access to the hard drive?
Is there any software that could do this?
Kind of, it will use Firewire instead of USB and doesn't work with the latest models of some Apple products as they do not have a Firewire port. Assuming you have a Firewire capable Mac and a PC with Firewire, you need to do two things:
1) Boot the Mac into target disk mode. This is done by holding "T" while the Mac is booting. This will present the Mac as a Firewire drive to whatever it is plugged in to.
2) Install Macdrive. This is an HFS+ (Mac filesystem) driver for Windows. Windows cannot natively read an HFS+ Volume. (There may be free alternatives, but I am not aware of them, I will continue to look and update accordingly.)