Hi cable2-
Thank you for getting my head thinking in the right direction. I went back through my network to see which devices were communicating with which other devices. The XP machine and the NAS were talking to each other, and the Win7 and Win 8.1 machine were also talking to each other, but it almost seemed like they were on 2 separate networks. However, all 3 machines were working with my network printer.
Long story short: I like everything about my router except for the fact that it only has 3 wired connections. The XP machine, the printer, the NAS, an old single-disk NAS, and a 2nd router upstairs (operating as a repeater) all needed wired connections, so I had an older router daisychained to the newer router as a source of more wired connections. This setup was working. I wasn't sharing any files among the computers, so I never really had a home network -- I was just using my setup to get Internet to all of the computers, to print, and to back up files from one computer to the old 1-disk NAS. In that role, it all worked fine. When I tried this week to set up a home network to share files, the computers didn't want to see each other. For now, I've simplified the system by removing the single-disk NAS and getting the XP machine on a wireless adapter, so now I just have my router's 3 ports going to the NAS, to the printer, and to the repeater upstairs. All of the machines now see each other (and the DNS-325). I am suspicious that the older router is not working properly. I just placed an order for a simple 5-port gigabit switch that I'm hoping will work more effectively to let me add more wired devices to the router.
Thank you for the links and for getting me to think about the problem from a more productive perspective. Though my setup still isn't perfect, everything that's attached to the network is now working properly and I think I have all the problems worked out. Thanks!