Are there any Mining OS to be started from USB? Considering my PC as a mining rig I don't want to install anything on HD. Setting up a system with full GUI mining capabilites (proprietary drivers, OpenCL, ...) can take some time. An out of the box solution would be nice.

Are there any preconfigured OS distributions (maybe Linux) containing all you need to boot and mine without further configurations?
LinuxCoin is probably the closest thing to what you're looking for. According to the wiki, Linuxcoin is:


  A lightweight Debian-based OS which includes in the image everything
  that is necessary to run as a Bitcoin client and everything needed to
  get started mining on ATI hardware.


Edit: It may also be worth looking into BAMT if your operation is of significant size

Update: How things change... BAMT is now the de facto mining distro. I'm now having a difficult time even finding a live link to download LinuxCoin from and BAMT has gone from experimental to a full-fledged mining distro usable for nearly any scale. I highly recommend BAMT over LinuxCoin.