Enabling Apache Module: Could Not Bind to Address 0.0.0.0:80

While in Virtualmin, I go to Configure Apache Modules in order to enable headers and expires. When I click Enable Selected Modules, I get this:

> Starting web server apache2 (98)Address already in use: make_sock: could not bind to address 0.0.0.0:80 no listening sockets available, shutting down Unable to open logs …fail!

My Apache server shut down by itself. I restarted it and now I notice that headers and expires seem to be enabled, which is good, but I'm unsure as to how that error message may affect my system.

How do I fix this?

__Ubuntu 8.04 LTS

Virtualmin 3.76

Apache v2.2.8__