You can still use the computer, it just means you need to purchase your own Windows license to activate for that machine (or put the Dell motherboard back in).
Story is complicated: in 2008 I have bought Dell's XPS 630i machine with XP/Vista OS as options. This year I have moved from USA to Poland and because my XPS was WAY too heavy I have only brought bits and pieces with me (procesor, RAM, Vista installation disk, graphic card etc.). However (and not really suprisingly) I have found that when I have installed Vista OS on a new (not Dell's) motherboard I got a message that I have to register my OS within 30 days. Trouble is that I have lost a note with my XPS 630 Service Tag and Express Service Code and all I have left is Vista product id and some 2008 emails from Dell to prove ownership. Any advice about how to deal with this situation would be greatly appreciated. Regards D. Przezdziecki
Transfer of Vista OS to a new machine