It's been fine for years. Suddenly, I cannot access the C:\windows directory
from another computer.

I've gone through sharing setup in both regular and safe mode;

The directory icon in Windows Explorer is the one with the 'hand'

IN the Sharing properties of that directory, everything is ghosted. But you
can see that "Share this folder on the network" is checked and has a Share
Name and "Allow network users to change my files" is checked.

The windows directory is on a Win XP Pro SP2 machine.

Trying to access from a Win XP Home Edition machine.

Connected through the same Linksys router for the past four years.