2
consider yourself lynched. XP, scary business.
Just as the error says, the permissions are not enough for the server to write to mainfile.php. You have to give mainfile.php full read/write permissions so the web server can write the config options. After the install you need to return the privs back to read-only.
I can't help with what you do to set the privs on XP though. I don't use Windows but the privs are what must be set at install time.