So, you want to Jailbreak your Archos 5 IMT?

This page will explain to you (in sequence) how to jailbreak your device in 3 tutorials.

The FIRST thing you need to do, is install firmware version 1.6.54
Archos made firmware version 1.7.02 and up (including 1.7.11) non-downgradable, so if you have a firmware version greater than 1.6.54, you'll need to downgrade your device using linux.
This is your first step.  The tutorial can be found here:

Now once that is done, or if you already have 1.6.54 on your A5, the next thing to do, is to install the DTi hack.

The DTi application that ships with Archos firmware 1.6.54 or earlier does not perform any kind of validation check on what it considers a valid game, so you can replace the game executable with anything and run it.

We can make use of this exploit to install patched firmware.
This is your second step.  The tutorial can be found here:

With the patched firmware installed on your A5, you can now install any firmware on your device, whether it is validly signed or not.
Now you can proceed to the final step -- jailbreaking your device.
This is your third step.  The tutorial can be found here:

This all may seem a bit daunting, but as long as you follow the steps to the letter, and read carefully, you'll have a jailbroken A5 in no time.