Plan on a "repair" installarion of XP, unless the motherboard is
"identical", not just "equivalent". See links for details. You need a full
retail copy of XP to do a repair, not just an OEM copy. If RAID, SCSI, or
SATA hard drives, you will also need to have new drivers for disk controller
on a floppy and hit the F6 key early during the repair. If plain IDE/ATA
hard drives, don't worry about F6ing.
http://support.microsoft.com/default.aspx?scid=kb;en-us;315341
http://www.webtree.ca/windowsxp/repair_xp.htm
http://www.extremetech.com/print_article/0,3998,a=23979,00.asp
http://www.geekstogo.com/forum/Repair-Windows-XP-t138.html
If successful, you will keep all personal data and settings and installed
programs. If not successful, you will need to perform a clean installation,
which means losing everything on the same partition as XP. Thus, backup
personal data before doing the repair. If necessary, attach hard drive to
another PC and copy files to CD, DVD, another hard drive (internal or
external), etc.
Activation should be straightfoward, if retail XP, and almost impossible if
OEM version of XP.
IP address is likely to change, if you have a normal home-user type internet
account. But, that has nothing to to with your new motherboard; most IPs do
not assign fixed IP addresses to home accounts, and powering off for a while
can result in a new IP address. If you have a cable modem, its MAC number
will not change, and that is usually how the ISP senses whether you are a
paying customer. What is on the home-side of the cable modem is usually not
important to the ISP, and with the right sort of firewall you could prevent
them for seeing it anyway. Ditto for DSL modems. If you have dial-up
access, then the IP address probably changes every time you connect.