List of Environment Variable found on Windows

Environment variables are a set of named values that affects the way running processes will behave on a computer depending on the variable stored. The environment variable with a standard name can designate the particular location on a computer system - this may vary from one computer system to another.

Default Values on Microsoft Windows

Variable	Windows XP/Vista/7/8
%ALLUSERSPROFILE% and %PROGRAMDATA%	C:\Documents and Settings\All Users C:\ProgramData
%APPDATA%	C:\Documents and Settings\{username}\Application Data C:\Users\{username\AppData\Roaming
%COMPUTERNAME%	{computername}
%COMMONPROGRAMFILES%	C:\Program Files\Common Files
%COMMONPROGRAMFILES(x86)%	C:\Program Files (x86)\Common Files
%COMSPEC%	C:\Windows\System32\cmd.exe
%HOMEDRIVE%	c:
%HOMEPATH%	\Documents and Settings\{username}
%LOCALAPPDATA%	C:\Users\{username}\AppData\Local
%LOGONSERVER%	\\{domain_logon_server}
%PATH%	C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;{plus program paths}
%PATHEXT%	.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.WSF;.WSH .com;.exe;.bat;.cmd;.vbs;.vbe;.js;.jse;.wsf;.wsh;.msc
%PROGRAMDATA%	%SystemDrive%\ProgramData
%PROGRAMFILES%	%SystemDrive%\Program Files
%PROGRAMFILES(X86)%	%SystemDrive%\Program Files (x86) (only in 64-bit version)
%PROMPT%	Code for current command prompt format. Code is usually $P$G
%PSModulePath%	%SystemRoot%\system32\WindowsPowerShell\v1.0\Modules\
%PUBLIC%	%SystemDrive%\Users\Public
{Drive}:\$Recycle.Bin	C:\Recycle.Bin
%SystemDrive%	c:
%SystemRoot%	The Windows directory, usually C:\Windows, formerly C:\WINNT
%TEMP% and %TMP%	%SystemDrive%\Documents and Settings\{username}\Local Settings\Temp %SystemDrive%\Users\{username}\AppData\Local\Temp
%USERDOMAIN%	{userdomain}
%USERNAME%	{username}
%USERPROFILE%	%SystemDrive%\Documents and Settings\{username} %SystemDrive%\Users\{username}
%WINDIR%	C:\Windows

Source: http://en.wikipedia.org/wiki/Environment_variable