Let's say I have a computer. It's a personal computer - the one you'd typically find in homes. In this scenario, it's running Windows XP SP3, although it could run 7, Vista, etc.
I have a USB drive which runs as a Linux Boot Disc - with something like Ubuntu, Puppy Linux, DSL...
If that USB OS is loaded into the RAM at boot, without any persistence (nothing is saved after each session), how possible (if at all) is it for malware to infect the main system (the Windows XP/Vista/7 system) that the USB OS is running off of?
Also, if a foreign computer is riddled with malware, is it safe to assume that booting the system to a USB OS will keep the USB OS safe from the Windows malware on the machine?