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.
A colleague of mine just had a similar problem last week. After extensive checking (even running Memtest86) it turned out that there was a problem with the filesystem.
To check, boot with your Snow Leopard installation DVD and run the Disk Utility to check the filesystem. If it does find errors, don't bother trying to repair the errors with Disk Utility, in my experience it can't repair anything. Instead, buy DiskWarrior and use that for repairing your filesystem. It saved me two weeks ago and my colleague last week, so while it's expensive it really is a life-saver, IMHO (no, I'm not affiliated with them in any way).
Best Answer
A few of things for you to try...
First, try starting in Safe Boot by holding the Shift key when you hear the startup chime. If you manage to successfully boot in this way and you can access the internet, download and install OS X 10.6.8 Combo Update V1.1. Restart and see what happens...
If that doesn't work - and you have the original discs that shipped with your Mac - start your Mac from disc 1, OS X Install Disc. Once it has started up find Disk Utility from the menu bar and select Repair Disk. When that has done select Repair Permissions. Finally, restart and see what happens...
If you installed from a retail copy of 10.6, follow the same steps above.
If you don't have the original discs, start your Mac and hold command + S to boot into single user mode. When it has booted you will see a command prompt, type in
/sbin/fsck –fy, press the Enter key when done. A disk check will then be performed. When that's finished, typeshutdown -r nowat the command prompt. Wait for it to restart and see what happens...