your trying to use ie the dvd player software have to tried to uninstall it and install it again. This is only presuming to have the software disc.
Try a system restore and see if that sorts it out.
Normally with dvd software programs they give you five chances to change the region, after the fifth dvd is played it will stay at that region and cant be changed unless you uninstall and the install the software program.
If your using windows xp and have the installation disc you can back up all yur data onto pen drive and do a complete reinstall on windows and install the drivers and then install all the software. B4 you do make sure yu save all your personall stauff on a pen drive and make sure you have the installation discs for the software.