Starting a couple days ago, the update manager appears to be failing. The menubar has that warning triangle, telling me that updates havent happened for 35 days, and when I open Update Manager, it tells me there are no updates to install . If I click on the "check" button,
I get the error window about failng to reach repositories, full error message follows below. I do have full internet access, i.e. web browsers, and apt-get run just fine. Any ideas? -- some corrupt files somewhere I need to delete?

** error list** W:GPG error:
http://dl.google.com stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A040830F7FAC5991, W:GPG error:http://packages.medibuntu.org precise InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2EBC26B60C5A2783, W:GPG error:http://extras.ubuntu.com precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192, W:GPG error: http://cran.r-project.org precise/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9, W:GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 531EE72F4C9D234C, W:Failed to fetchhttp://dl.google.com/linux/earth/deb/dists/stable/main/binary-power... 404 Not Found [IP: 74.125.226.194 80]
, W:Failed to fetch http://packages.medibuntu.org/dists/precise/free/binary-powerpc/Pac... 404 Not Found , W:Failed to fetch http://packages.medibuntu.org/dists/precise/non-free/binary-powerpc...404 Not Found , W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise/main/binary-power... 404 Not Found [IP: 91.189.91.14 80] , W:Failed to fetchhttp://us.archive.ubuntu.com/ubuntu/dists/precise/restricted/binary... 404 Not Found [IP: 91.189.91.14 80] , W:Failed to fetchhttp://us.archive.ubuntu.com/ubuntu/dists/precise/universe/binary-p... 404 Not Found [IP: 91.189.91.14 80] , W:Failed to fetchhttp://us.archive.ubuntu.com/ubuntu/dists/precise/multiverse/binary... 404 Not Found [IP: 91.189.91.14 80] , W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise-updates/main/bina... 404 Not Found [IP: 91.189.91.14 80] , W:Failed to fetchhttp://us.archive.ubuntu.com/ubuntu/dists/precise-updates/restricte... 404 Not Found [IP: 91.189.91.14 80] , W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise-updates/universe/... 404 Not Found [IP: 91.189.91.14 80] , W:Failed to fetchhttp://us.archive.ubuntu.com/ubuntu/dists/precise-updates/multivers... 404 Not Found [IP: 91.189.91.14 80] , W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise-backports/restric... 404 Not Found [IP: 91.189.91.14 80] , W:Failed to fetchhttp://us.archive.ubuntu.com/ubuntu/dists/precise-backports/main/bi... 404 Not Found [IP: 91.189.91.14 80] , W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/precise-backports/multive... 404 Not Found [IP: 91.189.91.14 80] , W:Failed to fetchhttp://us.archive.ubuntu.com/ubuntu/dists/precise-backports/univers... 404 Not Found [IP: 91.189.91.14 80] , E:Some index files failed to download. They have been ignored, or old ones used instead.