Windows Server 2008 is one of the most secure OS of year 2008
Studies conducted by IBM reviles that it is Windows OS which leads in terms of security over Apple OS X (MAC) or Linux karnel.
It is a long believe that, Linux and Apple OS X is much more secure than Windows based OS, where as reports published by IBM suggest opposite as IBM X-Force have found Windows Server 2008 more secure than OS X or any linux platform.

It is Apple OS X Server which is one of the worst in terms of disclosed vulnerabilities. However, do note, the report is prepared on basis of disclosed vulnerabilities, meaning there can be many more open loop holes in these OSes which are yet to be found that patched !!!
One may read the full report by downloading from IBM.COM -> PDF 4.58mb
Ramesh Kumar on February 9th, 2009
Its Windows Server 2008
Well congrats M$ !