I erased my hard drive on my 20' 2008 iMac and now when I try to install Snow Leopard from install disc, I get kernal panic "You need to restart your computer. Hold down the power button until it turns off, then press the power button again." Does it over

According to Mactracker, a 2008 20" iMac came with Leopard. Have you tried the original install disk? Perhaps the Snow Leopard disk is damaged.

Can you print the kernel panic report so that someone can determine with specificity what may be the problem. (not my strong suit).

Ciao.

Reboot the machine holding the option key down, when Startup Manager appears, click eject on the keyboard to eject the disk out of the machine.

Clean the bottom of the 10.6 disk with a little bit of rubbing alcohol and a soft clean cloth and polish, allow to dry streak free.

Reinsert and boot from the 10.6 disk (click the arrow), head to Disk Utility again and select your entire boot hard drive (drive makers name and size) and now click Erase, Security option > Zero All Data and then click Erase.

Wait for it to finish, it's going to take some time. Once finished, then quit and use the installer to install OS X.

If it fails, it's likely a bad, scratched or wrong disk for that machine.

The 10.6.3 white retail disks will work in that machine, no gray or black ones will work.

If the disk is bad, call Apple for a replacement copy.

How to erase and install Snow Leopard 10.6

For Snow Leopard Speed Freaks

If the next disk fails, I would look at a failing machine or boot drive, you can test this by installing OS X on a blank powered EXTERNAL drive.

.Create a data recovery/undelete external boot drive