If you had a Time Machine backup, meaning you had Leopard, you would be able to do a clean SnowLeopard install and then during setup, it would give you the option to migrate files over.

The you could select to have your user accounts and all your apps. migrated over, instead of creating new user accounts.

Of course, you need an external drive with the backup mounted.

One option would be to back up your drive to an external drive using something like Carbon Copy Cloner and have that drive mounted. I believe the migration section of the OS setup (which you get after doing a clean install and booting up) can detect user accounts and obviously move that and your app. files over. Pretty sure it lets you select every directory you want to migrate except System.

One note, the way the SnowLeopard DVD installs is tricky. It will start the install without first booting into the DVD. So you're still booted from the hard drive for the first part of the install and that means you can't easily do a clean install.

So you need to insert the DVD, restart and hold the C key down. Or just use Startup Disk in System Preferences and selected the DVD.

When you're booted in the DVD, the default is to do an upgrade install, not a DVD install. So you have to go to the Tools menu and select Disk Utility and then select the volume you want to install to and erase it, quit Disk Utility and go back to the installer.