You won't lose any file if you repair it.
To repair the hard disk:
1. Insert the Snow Leopard DVD and hold the C key while your Mac is starting to start up from the disc.
2. Choose your language, and go to Utilities menu (on the menu bar) > Disk Utility.
3. Choose Macintosh HD or your OS X partition in the sidebar, and repair it. If it could be repaired, go to > Restart. If not, you need to format the hard drive or replace it.
In this case, you need to make a backup of your files, thing I recommend you to do before repairing the hard drive. To do it, use an external drive and set up Time Machine > http://support.apple.com/kb/HT1427