Ubuntu :: Cannot Update 10.04 - Error During Signature Verification

Every time I try to update Ubuntu via the Update manager, I get the following error messages.
W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used.GPG error: [URL] stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A2019EA84E7532C8
W: Failed to fetch [URL]
W: Some index files failed to download, they have been ignored, or old ones used instead.

I already tried sudo apt-get update:
Code:
W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used.GPG error: [URL] stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A2019EA84E7532C8
W: Failed to fetch [URL]
W: Some index files failed to download, they have been ignored, or old ones used instead.
sudo apt-get upgrade

Code:
Err [URL] opera 11.00.1176
404 Not Found
Failed to fetch [URL] 404 Not Found
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
and sudo apt-get upgrade --fix-missing

Code:
Err [URL] opera 11.00.1176
404 Not Found
Failed to fetch [URL] 404 Not Found
I also tried changing my software sources server from United States to Main Server, but it didn't make any difference.