Server 2008 Web Edition limitations
From http://www.microsoft.com/windowsserver2008/en/us/editions-overview.aspx
Windows Web Server 2008 is designed to be used specifically as a single-purpose Web server, and delivers on a rock-solid foundation of Web infrastructure capabilities in the next-generation Windows Server 2008. Integrated with the newly re-architected IIS 7.0, ASP.NET, and the Microsoft .NET Framework, Windows Web Server 2008 enables any organization to rapidly deploy Web pages, Web sites, Web applications, and Web services.
There are some really good pages under the "editions" link at the top of that page as well. Specifically http://www.microsoft.com/windowsserver2008/en/us/compare-roles.aspx states that the Web edition does NOT support File Services.
In addition to the comparison pages on the Microsoft website, be sure to check the Licensing page.
Windows Web Server 2008 can be used solely to deploy Internet-accessible Web pages, Web sites, Web applications, Web services, and POP3 mail serving