It depends. I found that I did not have to reinstall windows, but I had to reactivate it because of Microsoft's "disadvantage to genuine customers" program. Then, I had to move the hard drive back to the old PC and call Microsoft again and explain why the installation changed again.
Usually, I order barebones as a budget solution and because it's easier installing a new CPU and transfering everything else than to start from scratch. Most barebones need a better power supply, so think of upgrading that as soon as possible.
The barebones I bought is in my signature, but the motherboard's being sent out for RMA after a month of use, but it's partly my fault and partly MSI's own Live Update program glitching on me.
The problem was that I used MSI's Live Update at 4 in the morning and didn't pay attention to what was checked. I had only meant to automatically update the chipset, sound and graphics card drivers, but Live Update also updates the bios for both their motherboards and graphics cards.
I just clicked yes without paying attention while watching the local weather and Live Update ended up flashing the motherboard's bios from within Windows. That's a new thing to me, the old way is the safest, booting to a diskette with a backup of the old bios being made before the new one is flashed.
Well, I was very apprehensive once I realized I'd gone into zombie click mode. MSI Live Update flashed the bios and then rebooted. The motherboard was dead and would not reboot. It was obviously a bad bios flash and I could not find a jumper on the motherboard to set it back to factory. So I RMA'd it and will put it back together when it returns.
A coworker did the same thing with an Intel motherboard and had no problems. I guess it's standard procedure nowadays to flash a bios from within Windows? That is the worst idea I've ever encountered in my 14 years of building my own PCs and it probably came about because PCs don't come with diskette drives anymore. I recommend USB diskette drives for bios flash and third party RAID driver installs.
If you're geting a different MB type, different memory type, don't even try.
My 2p
In my case, all I did was uninstall the old drivers and Intel utilities before transferring the motherboard, so I only had to use Windows Update after reactivating Windows.
There has to be a circle of Dante's inferno for the guys who designed the "disadvantage for genuine Windows customers".