My pc game called counter strike source will not work HELP?

hi my brother bought a copy of halflife 2 for the full retail price so it's a proper official copy. My brother installed it and i have windows xp home edition and the game works fine on his xpuser account, but on my xp user account it worked for about 2 weeks, then today when i clicked on the game and it loaded up half way, it said "your stam ticket has expired please type in password to continue" so my brother typed in the password but it just will not accept the password. And the game still works perfectly on my brother xp user account but not mine. Does anyone know what is wrong or can anyone help me please?