I suspect I already know the answer to this, since on PC's nothing ever seems to be easy, but here goes:
If I upgrade my PC by changing the motherboard, CPU, and memory chips, and keep all the rest, including the boot drive, will I still have to re-install all my applications?
Windows will throw some hissies and probably require you to reactivate it within 30 days. Some applications may detect the change in CPU and require reactivation. You shouldn't have to reinstall anything other than motherboard drivers.
As Kelly pointed out, you may have some activation issues accompanying the hardware changes.
If your O/S was the OEM version of Windows XP, you might have trouble with Microsoft, as they've changed the EULA over the years to the point where the OEM license is essentially anchored to a single machine, which basically means the motherboard/CPU combo. This isn't the case for retail full and upgrade versions of XP Home or Professional, which you could move to a new machine if it replaces your old one.
Wow, that's much better than I was expecting. I assumed I would have to start from scratch with Win 7, etc. Thanks.
I'm running a retail version of Win 7 Professional.
If you're going with XP, there are several sites that give helpful advice on changing the motherboard without reinstalling Windows and all of your applications.