Windows 7: Cannot access shared resources, used to work, Internet

I have a network with 2 Windows 7 computers (7100), some XP computers and 2 NAS devices (Lacie with embedded Windows) and 1 Mac.

The XPs and the Mac can acccess the two NAS-devices without problems. On my two Windows 7 computers I can see the other computers and the 2 NAS boxes - they show up in Network - but I cannot access them, I get an error message "\\NAS is not accessible....... The Network adress is invalid"

I can accces the NAS from Windows 7 thru my browser thru http: (using the name "NAS").
I can also connect from Windows 7 with Remote desktop Connection

This used to work, stopped on the 1st of September (my backupprogram has given an error message every day since then - I backup to the NAS device.)

I have turned of IPv6, I use a workgroup (not homegroup). I have Netbios over TCP/IP enabled.

I have the same problem on both Windows 7 PCs - and no problems on the XPs or the Mac.

I use Norton Internet Security (2010 Beta) and ThreatFire. Turning them off makes no difference.

Any ideas/suggestions?

Uninstalling ThreatFire solved the problem. I had tried inactivating it but that wasn't enough. I also uninstalled it from the XP machines but I'm not sure if that was neccessary (since the XP machines didn't have any problems). I also had to set up id's in the Credential Manager to be able to see the shares from the other computers - I wish there was a way to do this easily from the Networks folder.

I just Installed ThreatFire 4.6, no problems with this release