XP: new Mac OS shell?

According to Wikipedia the shell is a piece of software that provides an interface for users. The primary purpose of the shell is to invoke or "launch" another program, however, shells frequently have additional capabilities such as viewing the contents of directories.

And this is exactly how I am using Microsoft XP on my Mac. The new Parallels Desktop 3.0 turns XP into a piece of code that is almost invisible and that helps me to launch a few remaining apps that have not been ported to Mac OS X yet. I don't plan to upgrade to Vista ever. All I need is a small and stable shell. A piece of code...