Here is your problem.
Your environment variable settings were altered by some application you installed.
I just had the same problem and I know exactly how to fix it.
A previous poster said that you need to change your settings to c:\windows\system32.
This is not right as it will cause problems with installed applications and some native applications that windows has will not run if they are outside that directory.
Same location (Right click on My Computer, then select Properties, then Advanced tab, then Environment variables button located towards the bottom. You will be given a dialog that has 2 parts. The top part is for user variables. Usually there is not much there and sometimes will have nothing at all. This is normal and you do not need to add or adjust anything there. The second part is for the system itself. You will see several entries there. The one your interested in is the one with PATH label to the left labeled Variable. Left click to select the Path entry and then select Edit. You will be presented with another dialog with 2 fields that you can edit. Do not alter the Variable name field as the system needs that name to set paths for the windows environment. Under Variable value: I would never tell you to delete that line and reset it to a path that is C:\windows\system32. This will cause you problems if you had other paths set up for other applications there. What you do is copy the entire field so you can see what is all there using a text editor like notepad.exe. Once you copy the field the parts your concerned with are the entries %SystemRoot% and %SystemRoot%\system32. These should be the first 2 entries and in that order. Each entry is separated by a ";" (no quotes). So the first 2 entries should be like the following:
%SystemRoot%;%SystemRoot%\system32
There is a ";" (no quotes) between them. The first one is to tell the system where your windows Operating system is. You can type that in a command prompt and get something similar to C:\windows. This is important that it comes first as it tells the shell (A shell is the same as command prompt) and the desktop which is an application running called explorer.exe (not to be confused with Internet Explorer) Where the OS files reside such as the executables and command files are. The second is where the programs such as ping.exe and arp.exe are located.
Be careful when you do edit system stuff as you may break software and be forced to reinstall them.
Microsoft has a page that lists all the environment variable settings.
%SystemRoot%\system32;%SystemRoot% are only 2 of them. There are several more that serve similar to wildcards to the system folders and settings.
Here is the Microsoft page I am referring to:
Microsoft Corporation
Try typeing (without the quotes) "echo %homedrive%" in a command prompt window and see. The system is set up in a way that these wildcards point to the proper places.
I hope this helps you solve your issue.
Steve C.