I'm using an older XP computer for my DAW (currently too poor to upgrade) and started getting messages "the hardware on the computer has changed significantly. Due to these changes, Windows must be reactivated within 2 days".
 
Anyone know how to get around this? Yeah, I know support for XP has ended, but surely there must be a way to keep using it.

Do you have a serial? You should be able to click on the window when it pops up, and it will give you different options for activation. I always call the automated line, read off my serial#, then they give me an activation code. I've done it this way for XP and Windows 7. As long as you have a serial #, you should be able to get it activated.

Yep, the automated line should work just fine.

Yes, it does give me various options to use. 
I'll give them a call. 
My brother built this thing. Hopefully he still has the documentation.
Somewhere.

The product key is accessible within the working machine, but you need a way to decrypt it. See the article below.
 
https://www.lifewire.com/how-to-find-the-windows-xp-product-key-2626106
 

Thanks, Slart.
I have the original XP Pro install disk my brother used and the key is written on it.

Jeez, I was agonizing over nothing.
Call the number, enter some numbers in the boxes, click Next.
Done, despite the OS being unsupported for many years and the horror stories I read about trying to get it activated in the past.

I am still running an install of Win XP as a virtual machine, to give life support to some older applications that will not install or run on modern Windows.  It still takes a license key to activate XP, even when running as a VM.  I keep my old keys handy for that!