Portable solutions which combine a short-throw projector with a board are available but are still very costly.
There are a number of alternatives to the interactive whiteboard that give varying degrees of flexibility in the classroom.
The ebeam is a small device which converts any white area into an interactive board. It can connect to the laptop by bluetooth (short-range wireless) or USB cable. It provides a portable interactive board solution. RM price here.
A similar device is the Mimio.
If you want to control the screen from anywhere in the classroom, there are two choices – RM classpad or gyromouse.

The RM Classpad (and other similar devices) is a small tablet that lets you control the screen using a pen on the screen. It can be used with or without an interactive board.

The gyromouse is great fun. You control what’s happening on the screen using a wireless mouse which you wave in the air to move around the screen. It’s much easier than it sounds. This is a cheap but very effective way of controlling your computer from anywhere in the classroom.
Video here. RM prices here.