PPRuNe Pop,
I do not know if I would fall under guru, but I will do my best.
With WinXP (NT Kernal) it does not like changing motherboards. There are too many recourses for it to setup. In the end an new install of XP is your best bet. The Brand of motherboard means nothing to WinXP, only the Motherboard Chipset. You have a better chance of keeping WinXP working going from an ASUS A7N8X Deluxe (nForce2) to an MSI K7N2 Delta-ILSR (nForce2) than from an ASUS A7N8X Deluxe (nForce2) to an ASUS A7V8X (KT400).
Since you have two drives, I would make the fastest drive you have, your main drive. Windows is primarily Disk Access. You want your fastest drive to handle this job. I would put your Main Drive as Primary Master, Leave Primary Slave empty, your Second Drive as Secondary Master, your Optical Drive as Secondary Slave.
NTFS is a better file system than FAT32. (I know there is a lot of debate on this topic.) It offers the most robust file system and the security is much better than FAT32. (You can convert your FAT32 to NTFS without losing your data, but I would back up your data before you do it, just in case it does not work.)
As for the two questions you asked of me:
1) Some programs do not install anything in the windows directory or in your registry, you can just access them and they will work. Other programs need the windows registry and/or they install dlls or other files in your windows directory and they will need to be re-installed. When you do the installation of that program, just make sure you point it to the same location the original program was at and it will install on top of itself and work. The last one is programs like Outlook and Outlook Express, they need to have their data base either backed up (through its own utility) or exported (again though its own utility). Then you just restore or import.
2) You need to have the drivers for your motherboard (since the USB 2.0 Ports will need to be working) and/or WinXP SP1 (Since it adds USB 2.0 support to WinXP) for your comp to be able to access your modem. The software from the Modem should have everything it needs to setup your modem again (My guess is it uses PPPoE). You should make sure you have your username/password and any additional settings for your modem beforehand.
Good Luck,
Richard