I had the same problem on my Sony VAIO running Windows XP Professional SP3.
My problem is that the "PATH" environment variable did NOT include "C:\WINDOWS\system32"
Any executable in that folder could only be run from a cmd prompt if the command included the entire path.
Solution for me: add "C:\WINDOWS\system32;" to the start of my PATH environment variable, like this:
Log in with Administrator priviledges.
Right-click "My Computer" & select "Properties."
Click "Advanced" tab.
Click "Environment Variables" button.
In the "System variables" box, scroll down to "PATH" and highlight it.
Click the "Edit" button.
In the "Variable value:" box, add to the very beginning of the text "C:\WINDOWS\system32;" without my quotation marks.
Make sure you include the semicolon.
Click OK three times.
No reboot required. If this was your problem, then ipconfig should work right away.