Reinstall Snow Leopard and not lose data

I'm having problems with my iMac. Either it gets stuck on the gray startup screen or it gets to the blue screen and the icon keeps spinning.
I tried everything and all is left is reinstalling Snow Leopard.

The problem is I'm afraid to lose my files and data because I haven't backed up.
Am I able to reinstalling the OS without losing all my data files photos..etc

or am I gonna lose everything.

Thanks