Partial success: Now I can see the computer in the PC's sidebar (thank you!).
But when I tried to open a folder I was told I did not have permission to access the computer.
I went back to the Mac and made sure that all relevant folders had "read/write" permissions for anyone on the network. I restarted the computer.
That seemed to set things a step backward: Now when I click on the MAC in the PC sidebar I'm told "Windows cannot access \\\IMAC1114BB. Check the spelling of the name or see if Windows can diagnose the problem with the network."
That is not a name I chose for the computer, so I can hardly "check" the spelling. Diagnosing produced no answer.
My end goal here is to set up iTunes sharing.