OSX boot up question....

I'm really new to using Apple software, and will probably ask many questions in the future regarding it, hehe.... This isn't really a problem, just wanted to know if it's normal. I've been using my powerbook for about ten days now and so ar so good. I've just noticed though when I'm booting up that a small icon appears in the center of the screen which is the Finder logo, it flashes back and forth between that a question mark, it then after aboiut ten seconds continues to load OSX as normal, it didn't happen when I first loaded the software, only in the last two days. Just wondered if anyone had any ideas. Thanks - Phil