I've created a perfectly working clean up file with an XPlode GUI.

Can I call this cleanup file like a regular XPlode installer like this, but then under GUIRunOnce (in winnt.sif):

[GUIRunOnce]
UserExecute="%systemroot%\system32\Temp\XPlode\XPlode.exe"

or does it HAVE to be a .cmd file?

If so how can I call the XPlode file (XPlode.xml) via .cmd file?

Like this?:

cmdow @ /HID

START %systemroot%\system32\Temp\XPlode\XPlode.exe

EXIT

I think this will work (I call the clean up file cleanup.xml):

[GUIRunOnce]
"%systemroot%\system32\Temp\XPlode\XPlode.exe /x:#systemroot#\system32\Temp\XPlode\cleanup.xml"

Can I exchange "%systemroot%\system32" for "%system%"?

%system%, i would doubt. The forum for xplode would be your best chance for answers though?

Can I exchange "%systemroot%\system32" for "%system%"?

will you be defining said %system% somewhere? It does not exist otherwise.

I thought I read somewhere yesterday that you can use that string for the System32 folder in Windows XP. But I can't find that site anymore

As far as I know it doesn't exist.

try typing SET in your command prompt.. it shows all your environment variables, both custom and system.

You can however define one for yourself:

SET system=%systemroot%\system32

but then again, maybe %system% is a reserved name...