I met this error while updating.I read questions and answers about this problem , but the point is signing key. It doesn't return signing key!
Get:1 http://archive.canonical.com lucid Release.gpg [198B]
Ign http://archive.canonical.com/ lucid/partner Translation-en_US
...
...
...
Get:24 http://archive.ubuntu.com lucid-security/main Packages [380kB]
Get:25 http://archive.ubuntu.com lucid-security/restricted Packages [14B]
Get:26 http://archive.ubuntu.com lucid-security/multiverse Packages [4,556B]
Get:27 http://archive.ubuntu.com lucid-security/main Sources [118kB]
Get:28 http://archive.ubuntu.com lucid-security/restricted Sources [14B]
Get:29 http://archive.ubuntu.com lucid-security/universe Packages [125kB]
Get:30 http://archive.ubuntu.com lucid-security/universe Sources [34.5kB]
Fetched 12.8MB in 10min 33s (20.3kB/s)
Reading package lists... Done
W: GPG error: http://archive.canonical.com lucid Release: Unknown error executing gpgv
W: GPG error: http://archive.ubuntu.com lucid Release: Unknown error executing gpgv
W: GPG error: http://archive.ubuntu.com lucid-updates Release: Unknown error executing gpgv
W: GPG error: http://archive.ubuntu.com lucid-security Release: Unknown error executing gpgv