Here’s a few things:

%COMMONPROGRAMFILES% - System
Returns the location where Program Files for Common access are installed.
(C:\PROGRA~1\COMMON~1)

%PROGRAMFILES% - System
Returns the default location where Program Files are installed.
(C:\WINDOWS\Program Files)

%SYSTEMDIRECTORY% - System
Returns the location of the Windows XP system (.DLLs, etc) directory.
(C:\WINDOWS\System32)
may also be C:\WINNT\SYSTEM in Windows NT, or C:\Windows\System in pre-XP Windows

%USERDOMAIN% - Local
Returns the name of the domain that contains the user’s account.
>>> or
>>>The computer name if it’s not part of a domain

Registry Keys for Environment Variables

User environment variables:

HKEY_CURRENT_USER\Environment

System variables:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment

Use a REG_EXPAND_SZ value for environment variables that need to be
expanded, like %PATH%.

Setting Environment Variables in Autoexec.bat

Although Windows XP ignores the autoexec.bat file, environment variables can
still be set within it, though using autoexec.bat in this way is not advised.

%PROGRAMFILES% -> The default location of Program Files

Easier than reading it out of the registry.

It’s fixed now!