I can't fill in forms on web pages?

I cant fill in forms on the internet as some information is covered by the web page "bar" which is there to help you, for example if I access an airline's site and try to book a flight the dates are covered by the same bar I should use to slide up and down to help identify the date. the same applies anywhere where there is this type of "slide bar" can anyone help in simple terms? I am useing windows XP