Hi,

Maybee apache is already installed and running.

Try: Debian Squeeze Netinstall.
When at end of install you are asked about the role
of the system, deselect all.
This leads to a minimum system.
iRedmail likes a fresh, minimal system.
(eventually keep an image of the system prior to install iRedmail)

(iRedMail will fetch (nearly)all required packets itself.
Before installing iRedmail install the following packets via apt-get install:
ssh (if you want to access by ssh)
bzip2 (required for iRedMail install to unzip the distribution file)
mc  (midnight commander if you are a linux greenhorn like me...)

Greetings,

    Marcovaldo