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.
The Developer Tools (760 MB) included on the Snow Leopard DVD are not part of the OS X Lion installer. They are a free separate download in XCode 4.1
A whole whackload of printer drivers (~400 MB) are not included in OS X Lion. Presumably they are downloaded when a new printer is added.
The Snow Leopard DVD includes a Library folder (1.3 GB) with frameworks etc. needed because the DVD is bootable, while the OS X Lion installer creates a bootable partition for installation.
That said, the Essentials.pkg is actually bigger in OS X Lion: 1.35GB vs. 882 MB
Those are the major differences that jumped out at me, I'm sure there are other minor differences I've glossed over. The actual "Installation" package on the Snow Leopard DVD is ~4 GB, the OS X Lion installation DMG is 3.74 GB.
Best Answer
Aqua Extreme comes with a separate package called Remove Aqua Extreme which restores the default user interface.
If you download the latest Aqua Extreme from http://www.maxthemes.com/downloads/Aqua%20Extreme.dmg you will find the Remove Aqua Extreme package inside the installation bundle.
Run this package, it should restore everything.