Macbook keeps restarting

My daughter's Macbook, running Leopard OS 10.5), begins to start up (and does that for quite a while), never getting beyond the screen with the apple logo. After a while, it restarts and goes through the whole thing again.

I have "zapped the PRAM" a number of times. We are away from home, so I can't use my Disk Warrior disk, nor try reinstalling the system software, nor anything else. Does anyone have a clue about what is causing this? What to do? Reinstall something?

Thanks for any thoughts.

jenny