Well there are a lot of issues here, and most of my suggestions are based on a small pool of DYI and good bad personal experiences:
First, if you ar going from a VIA chipset to VIA chipset your mb drivers MAY survive if they are close in age. This may be the same w/ SIS to SIS ect, and AMD to AMD.. but I believe, if the MB are too different it could be problematic (as in definitely a problem).
The FAQ here helps to put it into perspective:
http://www.anandtech.com/guides/viewfaq.aspx?i=113
For legal/update issues:
Depending on OS and it's EULA, this could be a problem. Microsoft OS's are usually (standard OEM liscense in general) are married to the motherboard. Change MB and you are supposed to buy a new OS.....
As to my recommendation, just use his HD as "slave" so you can retain the data files (but lose all software functioning most likely). Reinstall or replace software onto new (old) box. Of course if you are keeping your old OS then this is not an option.
After doing the "master/slave thing and if the new box has enough free space, transfer old files over to the new c drive and then either pull old HD and stick it into an external case for backup, or store as a historic snapshot (of course in a few months it's value decreases).
NONE of this (except slaving the hd) pertains to anything older than Win XP....
Greater detail (both box specs ect.)is probably needed for more targeted advice