Common reasons why people migrate from IIS to Apache (and not the other way around) include stability, performance and security. This is partly because most people running Apache do it on an Unix variant (like Solaris, FreeBSD or Linux). Fortunately, Apache is multiplatform and runs on both Unix and Windows, offering a sensible migration path.
Common Windows based web development environments like Coldfusion or Active Server Pages have Unix ports or compatible environments (some are commmercial, some are freely available):
If you want to go for a complete open source solution and you come from a Windows background ( IIS + ASP + MS-SQL server) the roughly equivalent (and highly popular) combination is Apache + PHP + MySQL or PostgresSQL. You can learn more about PHP here
Support for Windows is greatly improved in the new 2.0 Apache version, still in beta stage at the time of this writing.