I recently had to use a rescue CD on a friend's laptop. I decided to use F-Protect as it was highly recommended by someone in the security field. I found that updating was not straight forward and required some experimentation.
If you are trying to use F-Protect and want to update it with the latest virus definitions, see this post
http://www.digitallachance.com/blog/2010/05/virus-definition-update-on-the-f-secure-rescue-cd/