Upgrading to Snow Leopard without optical drive
You will need a USB drive (8GB) and access to another Mac. You can use any external drive instead of a USB drive, but be aware that you'll have to format the drive and you'll loose any content on it.


Insert the Snow Leopard DVD and the USB drive into a Mac with a working optical drive
Start 'Disk Utility'. You should see your USB drive and the SL disk in the left pane.
Select the USB drive in the left pane.
In the right pane: Select Volume Scheme: 1 Partition
Format: Mac OS Extended (Journaled)
Enter a name for the USB drive in the 'Name' field. You can use "Mac OS X Install DVD".
On the bottom: click 'Options' and choose 'GUID Partition Table' and click 'OK', then 'Apply'. The USB drive will be partitioned and reformated.
When that is done select the 'Restore' Tab
There are two fields: 'Source' and 'Destination'
Drag the SL Installer DVD from the left pane to 'Source'
Drag the newly formated USB drive from the left pane to 'Destination'
Click 'Restore'
Once the restore is done eject the USB drive, put it in your Mac and reboot. Hold 'Option' while your Mac is booting, this will get you to the boot selection list. Choose your "Mac OS X Install DVD" and you're ready to go.