(Just documenting my experience for the benefit of the community / Google searchers experiencing the same issue.)
In my house, I had a Windows XP computer that shared a printer and some shared folders with the rest of the computers on the network using a Windows workgroup (i.e. not on a Windows domain; no Windows Server present on the home network.)
Other client computers in the home ran Windows XP, Windows 7, and Mac OS X Snow Leopard. Each client computer on the home network had identical username and password credentials set up to facilitate network sharing.
That all worked great. Then, over the holidays, I repaved the Windows XP computer that hosts the printer and shared folders by installing a fresh copy of Windows 7 Professional (x64). That's where the trouble began.
I started noticing a few days later that intermittently, the Windows XP and Mac OS X clients on the network were unable to print to the shared printer, and unable to access the shared folders offered by the Windows 7 computer. However, the other Windows 7 client computer had no problem printing or accessing the shares, even when the XP and OS X machines couldn't.
Rebooting the client machines did not help. Whereas, rebooting the Windows 7 computer hosting the printer and shared folders would make everything work again. However, after some time passed, the failures would recur.
What was the problem? I'll answer below.