Microsoft claims Windows 2003 Server is twice as fast as Linux, at least when it's used for file serving. I spoke to Jeremy Allison, head of the Samba team, who provided a few insights into the test configurations that don't leap out at the reader because they are hidden away in appendixes to the benchmark document. Allison feels this, in itself, is substantially responsible for the outcome
Source: http://www.linuxworld.com/2003/0509.barr.html