What is the difference between user variables and system variables?
Right-click My Computer and go to Properties->Advanced->Environmental Variables...
What's above are user variables, and below are system variables. The elements are combined when creating the environment for an application. System variables are shared for all users, but user variables are only for your account/profile.
If you deleted the system ones by accident, bring up the Registry Editor, then go to HKLM\ControlSet002\Control\Session Manager\Environment (assuming your current control set is not ControlSet002). Then find the Path value and copy the data into the Path value of HKLM\CurrentControlSet\Control\Session Manager\Environment. You might need to reboot the computer. (Hopefully, these backups weren't from too long ago, and they contain the info you need.)
Environment variables are 'evaluated' (ie. they are attributed) in the following order:
- System variables
- Variables defined in autoexec.bat
- User variables
Every process has an environment block that contains a set of environment variables and their values. There are two types of environment variables: user environment variables (set for each user) and system environment variables (set for everyone). A child process inherits the environment variables of its parent process by default.
Programs started by the command processor inherit the command processor's environment variables.
Environment variables specify search paths for files, directories for temporary files, application-specific options, and other similar information. The system maintains an environment block for each user and one for the computer. The system environment block represents environment variables for all users of the particular computer. A user's environment block represents the environment variables the system maintains for that particular user, including the set of system environment variables.