You can do it without reformatting and installing Windows. After you've swapped the boards, on the first bootup, boot to the WinXP CDROM. Go through the install process until it reaches the point that it locates your existing Windows installation, and tell it to repair the installation. It will essentially reinstall Windows, putting back all the default drivers and stuff, but leave all your non-MS application settings alone, and of course not remove any of your data files. When you boot into it again for the first time, it'll go through the usual hardware detection, and all the Windows settings will be back to the default.
You most likely have a normal NTFS or FAT32 partition set up. There's no way in Windows itself to change the size of partitions like that. You can get software like Partition Magic that will allow you to merge two partitions, or make one of them smaller and then the other one larger to use the newly free space. There are also other products that have the ability. PartitionMagic is somewhat expensive. Search google for "resize partition".