CentOS and Debian have each released End of Life announcements for CentOS 4 and Debian 5 in February.
Virtualmin will no longer be supporting those distribution versions, and we'd highly encourage you to upgrade to a supported version! Virtualmin tries to make it as easy as possible to migrate to another system:
http://www.virtualmin.com/documentation/system/migrate
Although CentOS doesn't support upgrades, it's possible to upgrade a Debian 5 system to Debian 6:
http://www.virtualmin.com/documentation/system/os/debian-lenny-to-squeez...
You can read the end of life announcements here:
CentOS 4: http://lists.centos.org/pipermail/centos-announce/2012-February/018462.html
Debian 5: http://www.debian.org/News/2012/20120209