Capturing a client PC.

I am a network administrator running win 2000 Pro on my desktop. I receive quite a number of support calls which invlove me having to move around which I believe I could handle from my desktop. How can I be able to take control of a users PC (shadowit so to say), be able to see its desktop and files and be able to rectify a problem on it all from my PC? Do I need to install a particular software, if so which one, and I need one that will not cost anything or not too much for our business?