help me understand file sharing and security and encryption windows 7

how does network security work with windows 7 and file sharing?
specifically...

a) say windows 7 pc1 shares out a folder called data. Not using homegroup or anything but user accounts and passwords- like i had done with windows xp. So correct me if i'm wrong, but on pc2 in order to see the data folder shared from pc1 in order to read and write to it I would need to have the same user account and same password on pc2. Or at the least know the username and password on pc1 for the account that the administrators has granted access to that data folder.

b) so if what i said in (a) is correct, my question is how are things secure when transferring data across the network? looking for specific answer here, and what you might set up in control panel, administrative tools, or gpedit.msc to make file transfer using windows 7 secure. so when i open a window and go to \\pc1\data\ and either copy or paste a file to that location, is that data being transmitted across the network encrypted or somehow protected? or can someone easily intercept that data? what is the accepted practice in this case for setting up file sharing so that data can't be intercepted?

c) how easy is it to intercept data on a network for unencrypted traffic? is it something to be really concerned about? i realize when the network is the internet then yes, but i'm referring to smaller lan's such as company/corporate networks.

d) all that said, when using samba file sharing from a linux server how does that fit into things regarding network traffic and encryption? can samba be configured to encrypt data traffic over the network, and be as secure as a windows7 share if windows7 does encryption? or is one more secure than the other... or does neither sharing mechanism do encryption so they are both the same?

a. having the same username and password isn't necessary but it does make things simpler.

b. Normally network data is not encrypted and could be intercepted with the right tools and knowledge. Encryption requires that both the sender and receiver are aware of the type and details of the encryption. Setting this up in a secure way is not trivial but has many implications. For that reason it is not done by default. You can configure IPSec but that is too complex to deal with in a forum post.

c. Interception of the data is possible. How big a concern this is depends entirely on the situation. It may be of critical importance, or not important at all.

d. There is no encryption by default. Don't know much about Linux.

Edit: My statements about encryption needs qualification. Wired networks are normally not encrypted. Wireless networks would be encrypted.