These are 'extras' for Ubuntu 11.04 ('Natty').
This release reached end-of-life recently, and is no longer supported for security and other updates.
See: Ubuntu 11.04 (Natty Narwhal) reaches end-of-life on October 28, 2012
If you need to be using a long-term-support release, you should be using 12.04 LTS (or possibly 10.04 LTS on very old equipment).
If you are currently using 11.10 (Oneiric):
Is there any reason you still have Natty-specific resources in your setup? Otherwise, removing these two lines will solve this error.
If you are still on 11.04 (Natty):
You should not be attempting to upgrade directly to 12.04, but instead first upgrade to 11.10.
Note: As you have not already done so, and may not be up-to-date, it would be better to do a clean install of 12.04 LTS instead (after backing-up all settings and user data). Any issues you may have with upgrading from 11.04 are not supported, as 11.04 is no longer supported.