My MOBO died and had Windows XP. It was so old, it's just easier to rebuild another system.
Like an idiot, I had not back-up HD's in a long time.
I don't mind buying a new OS- Windows 7 and a new SSD for clean install.
My question is this: After the new install, can I just re-hook up the old HD's and still have all my files?
Yes. Build the new system with SSD. Don't plug in the HDD yet. Install Windows on the SSD. After Windows is fully installed, install the HDD. Unless your HDD was damaged the files will be there. Make sure to set your device boot priority to the SSD and not the HDD since its unlikely Win XP will boot cleanly on the new system.