Kyder is right when he says "probably" and Les is right when he says to use the variables (IMHO)...

We have found a few machines in our orginization that would not run any programs from the command prompt that normally would. It turns out that some piece of software had improperly edited the path and caused the windows folder along with a few other folder to no longer be in the path.

Example: the %windir%\notepad.exe in this case would still work but notepad.exe alone would not along with any other programs in the windows folder.

Myself I try to use the Registry or SET variables whenever possible to point to the correct path, but on most machines it should work just by calling it as Kyder says.