If you declare a function to return a particular type of value (ie. something other than void), then you must ensure that all possible ways of exiting the function (implicit finish at the end of the function, or explicit with the use of a return statement) return a value compatible with the declared data type (eg. null would be compatible with both the string and Object data types).
Declare one variable and use it in return.
private bool Validate(clsUser objUser)
{
bool flag = false;
UserBLL objUserBLL = new UserBLL();
int intResult = objUserBLL.ValidateLogin(objUser);
if (intResult > 0)
flag = true;
else if (intResult < 0)
flag = false;
return flag;
}