Have a laptop running Win7 Home Premium, connected to a workgroup network. The Win7 laptop can't "see" any of the XP systems (or Linux systems running Samba either) on the network, but can still connect to them if you specify numerical IP addresses. It sees other Win7 systems just fine.
Workgroup name is correct, and the user name in question exists on all relevant systems. I've checked that Network Discovery is enabled. Turning off the firewall makes no difference.
This was working until a couple of weeks ago, and just stopped working for no discernible reason.