downgrade from 2.6.20 to 2.6.17

Open a terminal and give this command:

sudo apt-get install linux-image-2.6.17-10-generic=2.6.17-10.33

After which you should have it available in your boot menu.
Let us know if this doesn't work, with a verbatim copy of any relevant error.

No luck, perhaps I don have the repo set up correctly

ian@onegreenparker:~$ sudo apt-get install linux-image-2.6.17-10-generic=2.6.17-10.33
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package linux-image-2.6.17-10-generic
ian@onegreenparker:~$

Thats in edgy main. You just need to add this repository to your sources.list.

I added
deb http://za.archive.ubuntu.com/ubuntu/ edgy main restricted
to sources.list

Still getting an error:

ian@onegreenparker:~$ sudo apt-get install linux-image-2.6.17-10-generic=2.6.17-10.33
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package linux-image-2.6.17-10-generic

Did you do a sudo apt-get update first?

I´ve done the ´sudo apt-get update´ and now I´ve started the ´sudo apt-get install linux-image-2.6.17-10-generic=2.6.17-10.33´ and all seems to be working.

Thank you for your assistance, I´ve learnt something today.

The kernel downloaded and installed correctly, but is not available in the boot menu.

Can you show us the content of the /boot directory (ls -l /boot) as well as the content of your /boot/grub/menu.lst file?

By the way, I also found out that there is a newer kernel with security updates which you may want to try:

sudo apt-get install linux-image-2.6.17-10-generic=2.6.17.1-10.34

For this you need to enable the edgy-security repository too.