if you are getting that kind of screen. what i would do is try to bypass it , instead of playing along.
first try to boot into safe mode.
http://docs.info.apple.com/article.html?artnum=107393
if you are able to get into safe mode, that means your OS is still intact.
if not, then you should try booting to your install disc. run the disk utility. then repair disk. hopefully your Mac HD will be detected.
http://docs.info.apple.com/article.html?artnum=302672
( if you do not have 10.4.3 or later, the only difference is locating the disk utility. if you have 10.3.x then you can find the disk utility under the word "Installer" in the uppper left of the screen )
these are the first steps to getting past it.
