mike asked you if you were using winXP, but you didn't say anything so i will assume you are using win2000, winXP home or professional versions. this only applies to those 3 OS's. to only share with only users you specify and block all other users you must go to folder options and disable simple file sharing.
next you must add all users you want to share with in the user accounts in control panel. it is best to use the same name and password that your users use on their own computers.
example Bill has a computer with user bill42 and pass: hexmon40. you should create a user account on your own machine that is the same as that so bill does not have to remember 2 accounts.
also you must decide what kind of access they have. do not allow any user other than you to have administrative privleges. admins can do anything to the system without any restrictions. also you MUST turn off the guest user account if it isn't already.
after adding users you must right click and select properties on folders you want to share. since simple sharing is disabled you can now manually authorize specific users access rights to each folder.
example i have a folder called movies. i have 3 users: bill, sam, and jill. if i want to only give jill and bill access rights to the movies folder i would add their names to the user access list.
it will ask you to browse for users to add, these users must have a user account created on your computer. their names and password is what you spefified when the account was made.
i hope this was clear for you, although it is very late here and i am beyond exhausted. i will clarify any questions you have, happy file sharing!