It may be that you have lost the contents of your path command. Go to Control Panel > System > Advanced > environment variables and in system variables check that there is a value for path. It should look something like
c:\windows;%SystemRoot%;%SystemRoot%\System32\Wbem;C:\Program Files\Microsoft SQL Server\80\Tools\Binn\;C:\Program Files\QuickTime\QTSystem\;C:\PROGRA~1\COMMON~1\ULEADS~1\Mpeg
If not, add the lne in the format
PATH c:\windows;%SystemRoot%
and reboot.