Do you have Windows installed via Bootcamp? It usually appears when there is an error in your Windows partition...
If you have Windows in your Bootcamp partition, boot to Windows. Run chkdsk using command prompt. Or perhaps you run the Check Disk the next time Windows start. If you choose the second option, restart your Mac and boot right back into your Bootcamp partition.
Empty the Recycle Bin.. Now try to boot back to your Leopard..