If both boards have the same exact chipset, and the same exact IDE controllers, you should have no big deal keeping your previously installed OS. A few adjustments in device manager migth or might not be necessary.
If you are upgrading to a newer chipset type, you should try the following:
1) BEFORE removing your old mobo, boot into safe mode, then remove ALL disk controllers in device manager.
2) If you have any devices integrated in your old motherboard (video, sound, RAID etc.), you'd better remove them all. This is not a necessary step, but valuable if you don't wanna risk Windows using the old drivers for the new hardware.
3) If you are also changing other pieces of hardware, this is a good time for removing the old ones too.
4) Turn off the power and swap the motherboards, etc...
5) Boot Windows in full mode, have all drivers disks at hand. Better yet, copy them all to your hard drive BEFORE changing mobos.
If you follow this through, you could *probably* avoid reinstalling windows and all your software. No guarantee, do it at your own risk. There is nothing as too much caution, and backup, as always, is recommended.
That said, chances are, you'll be happy.