In creating a software restriction policy, I had to refresh on some of the Windows System Environment Variables. Google helped me find a site (http://www.wilsonmar.com/1envvars.htm) that listed quite a bit of useful information.

%SystemDrive% C:

%SystemRoot% C:\WINNT
C:\WINDOWS

%WinDir% C:\WINNT
C:\WINDOWS

%SystemDirectory% C:\WINNT\System32
C:\WINDOWS\System32

%ComSpec% C:\WINNT\system32\cmd.exe

%programfiles% C:\WINNT\Program Files

%Temp% C:\DOCUME~1\Usr\LOCALS~1\Temp from
C:\Documents and Settings\Usr\Local Settings\Temp
%Tmp%

%HOMEDRIVE% C: The drive letter associated with the user’s home directory

%HOMEPATH% The path to the user’s home directory (excluding drive):
\Documents and Settings\Guest

%OS% Windows_NT (even on Windows XP machines
The operating system the user is running

%USERDOMAIN% The name of the domain that contains the user’s account

%USERNAME% The user’s name

%USERPROFILE%

%USERPROFILE%\Desktop The user’s desktop folder