Insert boot disk?

Every time I switch on my computer (have XP), it tells me to put in the boot disk. I then press the little button for reset and then the computer starts fine. There is no problem (except that it takes a long time - at least 5 minutes before I can see the Windows XP screen). I have already tried switching positions on the mother board and then it did not work (from IDE 1 to 2) What should I do?