Switching motherboard without new install?

I need to replace my current motherboard with a new one, no other parts are changed.

Must I format my harddrive and reinstall windows, or can I use the old installation and let windows install the right drivers?

I realize, there might be a performance hit, but I'll probably need the new one just for a week and would like to replace it with the old motherboard again.

Windows may boot with the new board out of the box, but it is possible that you will have to massage it go get it to work. If it does work, you should still update all your drivers from the motherboard's web site. If it doesn't the easiest way to do it is re-install and pick "upgrade": This will keep all your programs intact, but replace the hardware drivers and any other faulty registry settings. Even after the upgrade, update your drivers.

If you try to upgrade to your same OS, but are not given the option (grayed out), it is likely because your original disk is older than your current service pack level. In that case, you will need to slip-stream your original OS disk to add your service pack level or newer. You can search how to slip-stream here as it comes up often.

It's dependent on a lot of factors here (mainly version of windows) and can be hit and miss no matter what. In my experience windows 7 will handle it as I took a hard drive from a laptop an placed it in an entirely different machine (intel dual core to an AMD dual core) and after a few restarts windows picked up all the drivers no prob.