If the wheel keeps spinning and you can continue to move the mouse around, chances are you're dealing with a dying hard drive. And if the laptop eventually completely freezes up and you can no longer move the mouse, that points in the direction of a bad logic board. The third possibility is a corrupted OS install, or a system that is so bogged down it can't successfully boot up. Try booting in safe mode by holding down the shift key immediately after hitting the power -- booting with minimal resources may improve the laptop's ability to get through the boot sequence.
Yet another possibility is that it doesn't have enough RAM installed to boot. Have you ever witnessed this computer working, or has it been this way ever since you've had it? If it has booted up in the past with the amount of RAM installed, then this is not likely the problem. But if it's been stripped of RAM and therefore only has the 128-256MB on the board, that might explain it struggling to boot up.
The best way to proceed is to wipe the drive and see if it will let you reinstall the OS. If that works, you can write it off as a software problem. If you hit roadblocks during the install process, it's likely either a bad drive or board.
If you need to save your data before re-installing, you can put the machine in target mode by holding down "t" while powering on and connecting it via firewire cable to another Apple computer. The hard drive icon should appear on the desktop of the other computer, and you can then copy files over.