Can Upgrade - Boolean issue

message = ""

def can_upgrade(m, s, os):

    if ((m >= 1) and (s >= 2.6)) and os == ('Windows 8' or 'Windows 7'):
        message = "you can upgrade"
        return message

    else:
        message = "you can not upgrade"
        return message

def main():
    #  Gather inputs from user
    memory = float(input("Memory:  "))
    speed = float(input("Speed:  "))
    cur_os = input("Current OS:  ")

    print(can_upgrade(memory,speed,cur_os))


main()