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:
Although CentOS doesn't support upgrades, it's possible to upgrade a Debian 5 system to Debian 6:
You can read the end of life announcements here:
Debian 5: http://www.debian.org/News/2012/20120209