It is the DVD ROM that has the restriction, not XP.
You can override the multi-region restriction from most DVD players. Just Google the model number and search.
Easier is to do as I do and have a second (or even more - there are four region settings) DVD ROMs on a machine reserving a region for each one. It won't cost a lot, you can get a DVD ROM for a few quid, and doing this is safer than fiddling about doing firmware alterations to your machine.