Win7 stopped seeing network shares on XP computer

A client's Win7 laptop (A), connected wirelessly via a router, stopped, around two weeks ago, to see network shares on a XP computer (B). It still accesses the internet, gets emails, etc, so we know that it is on the network correctly. See below (summary) for further proof of its connectivity.

As part of troubleshooting, I hooked up my Win7 laptop (C) wirelessly to the network. My laptop, C, does access the shares on A. B accesses the C laptop as well.

Summary: (All computers have network shares.) B and C can ping each other. B can access shares on C. B can't ping A or access shares on A.
C can ping B. Can't access shares on B but can see the media sharing of B. C cannot ping A but it can access shares on A.

I have checked the settings on B, to make sure file sharing is on, make sure it is on a Work network, the encryption setting, etc.

What are the troubleshooting steps to take?

Thanks!