Is there a mininal, command-line-only distribution of Ubuntu or similar that runs within MS Windows

You might find that something like coLinux or andLinux might be slightly lighter but personally, I'd choose a VirtualPC/VirtualBox/VMWare virtual machine every time.

The problem with the xming-style native builds of Linux distributions is they're very non-standard. They're hacks to serve a purpose. They might fit your purpose but I prefer to have standard things available. With a VM you can port to a proper bare-metal install if the need ever arises.

If you are only looking to run some Linux commands, you may want to try Cygwin . Cygwin, it is a Unix/Linux environment for Windows that can be used to have all that tools that windows lacks: bash, sed, awk, grep, vi, tail, ps, and many more. Once installed it will provide a shortcut that opens a terminal with a bash prompt. It also has an X-server environment if you need it. I've been using it for a few years now and it makes my live easier in Windows.

Sounds to me like you should look into VMWare - as for a "minimal, command-line-only distribution", take a look at Ubuntu Server.