OSX 10.6.8 Clean install

just download the software from the app store, then create a usb boot drive. Alternatively, if you have the recovery partition (which I think was included in a later software, like 10,7 or 10.8), reboot the computer, hold commanf and R until you hear the sound, then you can release. when it gives you the options, choose disk utility, then format the primary hard drive, then install the OS.

Also, just remembered that if you don't have the recovery partition, you should have the included recovery disks. No matter what you do, just make sure you back up anything you want to save, and having fast internet helps if you go with the recovery partition route.

Where can I find out if there is a recovery partition?

restart it, hold command + R and if it works you have it I guess. You will need to redownload the OS, so sometimes making a bootable usb is better because you can still use the computer while it is downloading

Seconded on the bootable usb drive. You can look up what the terminal commands are. Usually you download the .iso, convert to .dmg using one command, unmount the drive with diskutil unmountdisk /dev/disk# then write the .dmg to the usb. It's easy, and well documented. run disktutil list to find out which /dev/disk# your usb is. It'll be /dev/disk1 or 2 at the end or something.

You don't need a bootable drive.

Just hold down Command+R on startup. Go to disk-utility, click on the hard drive, go to 'Partition', in the drop-down box switch from Current to 1 Partition (or however many you want). Click 'partition'. This erases all data. Then, when you close disk-utility, you've got the option to 'reinstall snow leopard'. From there it's all straightforward.

I've literally done this twice in the last 12 hours.

Oh, I'm stupid. The first poster already mentioned this. I don't know if above method works with Snow Leopard, but I presume it does...