Home-group Enter Network Password?

I've been through pages and pages of Google and Bing searches, and I've found plenty who have this issue, but none who seem to get it solved. Three computers, all running Windows 7 Home Premium 64-bit, are members of a Homegroup that functioned perfectly for more than a year until Tuesday of this week. On Tuesday, I installed Windows updates and also updated from Norton 360 v5 to v6.Computer A and B don't have password protected logins to Windows; Computer C is password protected. Until Tuesday, A and B have never been required to enter a password to access shared files and folders on C. Computer A still communicates fine in both directions with B and C, without having to enter C's Windows login credentials. B can be accessed by C, but when B attempts to access the C libraries that are shared by C to the Homegroup, the appropriate shared folders are shown (Documents, Pictures, etc.), but instead of their usual icons, the icons look like this: url... and if I try to click on them, I am prompted for a Network Password like this: url... The name following "Enter your network password to connect to:" is the name of C, and the "Domain" shown below the password block is the name of B. (Domains aren't even possible with Home Premium, are they?)

If I enter C's username and password I can access C from B, but upon rebooting B it starts all over again, even if I click "Remember my credentials." Sometimes I don't get the prompt and instead am told that I don't have permission to view