I am running a server using Debian 6.0 squeeze. I host about a dozen websites (virtual hosts) I installed Webmin the other day, and then I installed Virtualmin GPL manually. I soon discovered that wehenever I rebooted my system MYSQL would not be running. I had to go into Webmin to manually start it. Virtualmin had a number of complaints. 1. It said I should add 127.0.0.1 to DNS list or to disable BIND to DNS. I tried both. When one error was fixed, another error popped up. It seemed like it never ended. I tried disabling almost everything in Virtualmin... still had the MYSQL problem... had to start it manually after every reboot (never happened before installing Virtualmin - everything started fine) Finally, I uninstalled Virtualmin, and left Webmin on the system... now MYSQL starts at boot as normal.
I am debating whether I should try installing Virtualmin again with the automatic install script. But I am afraid that it might corrupt my server if I do. The last thing I want is for it to corrupt, or change email databases, Wordpress databases, apache config files...etc...etc..
Should I give it a shot?
You definitely should not run the installer script if your system already has Apache+Co. installed and live websites on it! You'll quite probably make things much worse with that.
Same goes for manually installing Virtualmin really, i.e. on a live system. It is possible, but only recommended if you are a Linux expert, know your system and software really well and are prepared for extended debugging sessions.
In any other situation, a clean install on a new system, and migration of the existing sites (i.e. copy webspace contents over, dump and re-import databases, create mail users and copy mails over, ...) is recommended.