If on opening a command line prompt and trying to run something like ipconfig or ping, you receive an error message reading something like ‘ipconfig’ is not recognised as an internal or external command, chances are pretty good that you somehow messed up your PATH system environment variable.
To fix is pretty simple though: Simply add ;C:WindowsSystem32 (or whatever the path to your System32 directory is) to your PATH variable.
If you are using Windows Vista, right click on the My Computer icon and select properties. Click on the Advanced System Settings menu item on the left of the resulting dialog and then click on the Environment Variables button at the bottom of the subsequent dialog.
At the bottom of the Environment Variables dialog should be a section entitled System variables. Locate the PATH variable (if it doesn’t exist, create it) and add ;C:WindowsSystem32 to it. (Note that mulitple paths are delimited by a semi-colon).
Save your changes by closing the dialog box, relaunch your command line prompt (this will load the new system variables) and try again.