There are two levels of region protection here. The first is at the drive level - this would lock a drive to only play certain regions, which you have corrected.
The other level is at the operating system/software level. XP has it's own limit in place to stop you changing as it's "Media Player" can play DVD's.
To get around this, you need a region free utility. Try click here as a start, or try searching for "region free" at some download sites ( e.g download.com) or search engines (e.g. google.co.uk )
This kind of software sits in the system tray and will kick in when you load a DVD, therefore allowing you to play any region.