What do you mean without rs running? Rs has to be running to bot it.. You mean the OS itself only use 300mb? CentOS is a good one, but 300mb is pretty limited. I'm sure you could find some ubuntu distros that have been stripped to run very quickly. Damn small linux runs on 50mb using a live disk, but I dont think thats what you want in a virtual machine. But check out DistroWatch, i'm sure youll find what you need.