Username & pawowrd create for excess LAN

i have 5 pc and connet through LAN and date & folder use /excess through Lan
i want to creat a password on shared drive in win xp sp2 for lan users cannot access without a password

how can we create user name & password in lan
if any boby use/excess another Pcs alway ask or first time ask username & password. please tell me how we can create username & password .& security.


bye