Florida researchers found in their study, windows web server is more secure than a similarly set-up linux server. They compared Windows Server 2003 and Red Hat Enterprise Server 3 running databases, scripting engines and Web servers. Their criteria included the number of reported vulnerabilities and their severity, as well as the number of patches issued and days of risk — the period from when a vulnerability is first reported to when a patch is issued. On average, the Windows setup had just over 30 days of risk versus 71 days for the Red Hat setup, their study found.
More details at source....
Lots of comments about this study at Slashdot.