Back to Snow Leopard from Lion install method
Read and print out these instructions, your computer is going to be offline and you wil be cutoff from help until your machine is restored.
Clear the Desktop, Downloads and Trash of anything you wish to keep by placing their files in the respective Documents, Music, Pictures, Movie folders.
Disconnect other drives except the sole backup drive.
Backup ALL your Users folders (Documents, Pictures, Movies, Music etc) manually (drag and drop methods) to a (not TimeMachine) external powered drive (HFS+ journaled formatted in Disk Utility) and disconnect, your going to be wiping the entire boot disk of ALL DATA.
(warning, everything will be gone and not recovered, OS, programs, files, Windows etc all gone.)
Note: You might want to hold c and boot off the 10.6 installer disk and use Disk Utility to format the new blank external drive instead of using OS X Lion if that's hosed. Then reboot into Lion and copy files, may be safer that way.
Here we go!
Hold c and boot off the 10.6 installer disk that comes with your computer and second screen in just STOP there, don't install OS X yet.
Look at the Utilities Menu for Disk Utility.
On the left is the name of your hard drive maker, click it and Erase (format HFS+ Journaled), give it the same drive name as before, and click Erase...
(note: if you want to "scrub" the drive of old files that haven't been overwritten yet, then use the Security Option > Zero Erase, takes a lot longer)
This should wipe the drive of ALL partitions (GUID, OS X and 10.7 Recovery, Windows if present)
When it's done, quit and install OS X 10.6. Then install all your programs from fresh sources and validate/update.
When you setup a first account, use the same user name as before, this way you can simply drag and drop the content of your previous Users folders from the external drive right back into the new Users folders and everything should work peachy. Links in iTunes to music, playlists and iPhoto links especially.
Update OS X to 10.6.8 using the Combo Update for best results.
http://support.apple.com/kb/DL1399
(Note: If your original machine had 10.5 and you want the free iLife that comes with the disks with the computer, then you'll have to install 10.5 first using the same c boot/erase/format methods as above, then update to 10.6 via the disk, then Combo Update 10.6.8)
