To install SharePoint on a Windows Server 2008 machine you will need the SharePoint install that includes SP1.

but… do this first:

http://blogs.technet.com/wbaer/archive/2007/10/30/installing-windows-sharepoint-services-3-0-on-windows-server-2008.aspx

fun?

x86:

http://www.microsoft.com/downloads/details.aspx?FamilyID=ef93e453-75f1-45df-8c6f-4565e8549c2a&DisplayLang=en

x64:

http://www.microsoft.com/downloads/details.aspx?familyid=9FB41E51-CB03-4B47-B89A-396786492CBA&displaylang=en