The van Nistelrooy family is from the village of Nistelrode. This is the origin of the surname. My guess would be that Nistel means 'nestle', and 'rode' in toponyms means 'wood, small forrest' (see also
http://www.dwb.uni-trier.de/index.html ).
Variants: Van Nesselrooij, Van Nestelrode, Nestelroij, Niestroij, Van Nisselroij, Van Nisselrooij, Nistelrode, Nistelrooij (and/or with y for ij).