During the order process you can choose which operating system* to install on the server from the following various options:

Linux CentOS 5.x 32bit
Linux CentOS 5.x 64bit
Windows Server 2003 Standard 32bit
Windows Server 2003 Enterprise 32bit
Windows Web Server 2008 32bit
Windows Web Server 2008 64bit
Windows Server 2003 Professional Edition

According to the chosen operating system, the server will have different features:
Windows server features
Linux server features

Alternatively you can specify a Linux operating system of your choice, for which minimal installation will be performed (for Linux systems which are not free the purchase of the license will be down to the customer).

* the availability of the above operating systems is subject to the type of server that is  purchased.