about windows 2k IIS

when i am installing a software ,setup program tell me that your os need to install IIS 5,but i have already install it.
i find an question,if i install os and the same time istall IIS 5 ,that install is right,no problem.
but if i havenot install iis in the first install os then after i add IIS programe in windows components of control panel,that software still tell me ,i need install iis.
why?how to solve the question,thanks.