Hi Tambu,
there's a couple of innaccuratenesses in your post.
First, it's not a matter of whether an app uses "Windows Networking" (how you called it) or not. All apps access the network in pretty much the same way. The difference is, however, where from. Logon sessions are associated with so-called desktops. I.e. logged on user, his workspace. A system service runs on a different dekstop than the logged on user. This is why the avast service is unable to access the share even thought you have already filled in the credentials (on your desktop).
But to discuss the actual problem: I'd say that it's not too relevant whether guest access is enabled or not on the share. The interesting part is, why is the networking manager asking you for username/password in the first place? This is not how it should work - it should open the share automatically, without the need of reentering the logon data every time you start your machine. That is, it should work automatically. Of course, only if the account under which you're logged on on your machine has access to the share (not a different account). Please note that you must have a computer name specified as part of the logon name when specifying the access control list on the server. I.e. COMPUTER\Account, instead of simple Account. Otherwise, the name Account will be considered as an account called Account on the target, not on the machine from which your making the connection.
Can you check the ACL's on the Samba share and verify this?
Vlk