have a look at This Linky
windows networking was originally set up in the old days as a 'trust' type network where security was minimal and a simple password system was all you needed for folder shares, and users didn't know about admin shares. pre-internet so your lan was generally NOT connected to the outside world, except maybe thru machinations of your sysadmin via more secure software and hardware.
Windows 7 is a bit better, as i believe it doesn't automatically allow local access to the admin shares (if the other pc's are not Windows 7 they will in general allow access any admin account on the net), you need to go thru the network and sharing centre where you can elect to password protect shares. i personally turn off the windows networking, netbios and netbios services, file and printer sharing, etc (and block netbios in my software firewall) so can't offer a complete answer as i can't test it...)
if you can connect to OTHER peoples shares, it's because they have not disabled or password protected THEIR shares. up to them (or the sysadmin) to do that on each pc. windows networking works by shouting each pc's name, address, and share info to any other pc that cares to listen so they know you are there. adds a lot of extraneous info bouncing around the cabling if you don't want or need it. another reason i turn it off, to save a few resources & cpu cycles.
google on 'admin shares password Windows 7' - lots of info out there, being in a home group may disable access to admin shares on your pc by others. i have disabled admin shares on my pc entirely (per linky instructions).