I had the same problem with Fedora 3, and "yum update" fixed this for me.
To get "yum update" to work I had to :
1) Set the GPG key for the rpm program (see yum documentation on fedora.redhat.com). This is done with the following command:
rpm --import /usr/share/rhn/RPM-GPG-KEY-fedora
2) Set the "http_proxy" environment variable for our proxy server (also in yum documentation). This is done with:
http_proxy="http://server
ort"
export http_proxy
(note that the shell looses this setting when you log out)