Problem starting my macbook pro

My second hand macbook pro will not startup properly.

When I turn it on, it wont get past a grey screen with an icon in the middle that flashes.

The icon is a file icon with a question mark over it.

I dont get any other options.

Does any know what's wrong?

Thanks in advance.

Same problem here with a MBP Late 2008 that I just installed the 10.5.6 update on to. Flashing folder with a question mark. I can get it going by restarting again by holding down the "option button" during the chime, and then select the OS X startup disk, but this is far from a fix for this increasingly common issue. Anyone have any ideas?

Well, I went into system preferences and checked startup disk. Turns out, neither the Windows XP Bootcamp disk, nor OS X were selected. I selected the OS X internal HDD and rebooted and now it seems OK using the "restart now" popup. I've now tried it repeatedly with the AC adaptor hooked up as well as just on battery power - OK. I've also tried complete shutdown, wait 60 seconds, and manual push button restart - OK Finally, I tried just the plain old "restart" option from the Apple menu - OK. All seems to work now. We'll see if it continues...