How to download Snow Leopard

The above [now below as this is the 'accepted' answer] instructions will install Snow Leopard - but not technically a clean install.

To do so follow the above steps but enter Disk Utility after choosing the language in the installer. You can wipe it from there.

Stolen from Macs.com

the Snow Leopard installer will ask which language you want to use as the main language. Make your selection and click the right arrow key.

The Install Mac OS X screen will display. Click the ‘Utilities’ button.

In the Apple menu bar, select ‘Disk Utilities’ from the Utilities menu.

Disk Utilities will launch. Select one of the following instructions, depending on what you wish to do.

Erase a Volume. Use these instructions to erase an entire volume. Be sure to back up all of your data first.

Format a Hard Drive. Use these instructions if you wish to erase an entire hard drive, including any volumes/partitions it may contain, and not create any new volumes/partitions. Be sure to back up all of your data first.

Partition a Hard Drive. Use these instructions if you wish to erase and partition a hard drive. Be sure to back up all of your data first.

When you have finished using Disk Utility, select ‘Quit’ from the Disk Utility menu.

You will be returned to the Snow Leopard Installer to continue the installation.

This has happened to me in the past. The solution that worked for me was making sure that the disc surface is clean and scratch free, and I mean very clean. Using normal CD wipes is good to get dirt off, but if it is scratched more than slightly, then you may have problems.

Or the disc may be defective from the factory, it does happen and they do exist.

Try talking to your local Apple Store about a possible replacement, or if worst comes to the worst, you can order one here (Apple still stocks them).

Hope this helps!