If you still have the Snow Leopard DVD, then boot from it by booting while holding down the C key. Then choose Disk Utility. When the disk utility window is open make a note of the S.M.A.R.T. status in the lower right hand corner. If it is anything other than "verified", your hard drive is dying and needs to be replaced.
Next choose Repair Disk and see if Disk Utility can repair the disk. If it can and the S.M.A.R.T. status is "verified", see if you can reboot normally.
If the S.M.A.R.T. state is not verified, shut down and do not start up again until you have an external drive available to make a back up of your drive.
You can download the old version of Carbon Copy Cloner, ver. 3.5 to clone your drive:
http://bombich.com/download