MS SQL Server uses its own (not the OS's) thread scheduling system.

see
http://download.microsoft.com/download/1/2/8/128875ad-df12-4e9e-88e0-250f79b85eee/IntlsDeepDive.ppt
http://builder.com.com/5100-6388-5144021.html

and in SQL Server 2005 it even has its own mini "operating system"

http://blogs.msdn.com/slavao/articles/441058.aspx
http://blogs.msdn.com/slavao/archive/2005/02/05/367816.aspx

whether this means Windows is a poor OS for not taking care of these
things or MS SQL Server is a good dbms because it doesnt rely on the
underlying OS is a matter of opinion ==)

What direction Firebird will evolve to and how it will solve these
kinds of problems is a matter for Firebird x, where x > 3, and for the
Firebird luminaries to debate on. I eagerly anticipate the good show ==)

[Unedited top-posting trimmed by moderator]