The answer is maybe... (Win XP)
I recently did just that replace the mobo and 'recoverd' the windows installation and all data on the fixed disks.
See this thread
http://www.subsim.com/radioroom/showthread.php?t=171134
The key is to have a windows CD boot from that and do a recovery...
There is a precise set of steps to take I will try to find the article and link it here.
BTW you will have to get all the freakin service packs re-installed which takes hours.
I would have simply reformatted but dident want to loose nearly a terrabyte of data.
http://michaelstevenstech.com/moving_xp.html