Get your Windows CD ready cause you might need it. Because your two boards are both 845 series you should have hardly any problems doing the switch. Go ahead and replace the motherboard and let the computer boot and attempt to detect and install all the additional/changed hardware that the new motherboard has.
If you can't get it to boot into Windows the first time then restart the computer and boot from the Windows CD and choose Repair. Once the repair process has finished you should be able to boot into Windows. If you still have trouble, come back here and let us know what issues you have.
Your best option of course is to reinstall Windows with any motherboard upgrade, but I can understand your desire to keep all your installed apps.
techfuzz