Start>Control Panel>System>"Hardware" tab>"Device Manager" panel>Device Manager>DVD/CD-ROM drives. Locate and select the appropriate device (your DVD-ROM drive); click to highlight it. Right-click on the highlight, select "Properties", select the "DVD Region" tab and set whatever region you want. Note: Pay attention to the warning before playing with this; the 4th change you make is the last one that drive gets; ain't no more, and ain't no going back.
DVD Regions
Some DVD-ROMs and DVD players can be hacked, either with software/firmware modifications alone or via hardware modification, or both. Doing so will void any warranty you may have. Your toy, your call, your risk.
DVD-ROM Region Hacks
DVD Player Region Hacks
(Note: No endorsement implied, use at your own risk. Some folks have encountered significant inconvenience, major disappointment, considerable dismay, and additional expense. Enjoy.)