Wierd characters. How can I avoid them?

Hi! My guestbook on www.filaung.no (–> gjestebok --> les ) prints wierd characters like: Ã, ¥, and so on every now and then. Its a norwegian site so I use international characters, and when ever I print a message from my computer it’s no problem, but it seems like some computers mess the international characters all up…

Does anybody know what causes this to happen? And/or how I can avoid it?