That Suid warning is normal and nothing to worry about.
However, to clean install:
Insert your SL disc, and restart holding the C key as soon as the start up chime sounds.
Choose language, and click continue.
Choose Utilities>Disk Utility in the menu bar at the top of the screen.
Choose Macintosh HD in the left column, and click erase tab.
Format should read as Macintosh OS extended (Journalled)... and if so, click erase.
This will take a few seconds, and once done, quit disk utility, and continue with installation.