SharePoint Server 2007 RTM or WSS 3.0 will not install on Windows Server 2008 as it requires SP1. In order to install MOSS 2007 on Windows Server 2008 you will need to install 'MOSS with Service Pack 1' (this includes WSS with Service Pack 1). This is known as a slipstream installation and it contains the original image for MOSS/WSS RTM and the MOSS/WSS SP1 files. The problem is that this 'all in one' slipstream for MOSS was not available for download, however now it seems that this has been released and can be found here:
Office SharePoint Server 2007 with SP1 (32 bit)
Office SharePoint Server 2007 with SP1 (64 bit)
This seems odd as the Install is about 600mb and states that it is a trial, however bear with it, you will need to register for download, and you will find 2 executables (OfficeServer.exe 295.0 MB and OfficeServerwithSP1.exe 409.2 MB) (What would we do without broadband!).
Install the OfficeServerwithSp1 executable.
Initially you will be asked to enter your Product Key. If you have an official product key you can use that, otherwise you should have been sent a trial key.
Now you have a SharePoint Server that will install on Windows Server 2008, with no problems at all. Microsoft have not yet released this as a bundle, or even on MSDN, however I am sure they will do this in time.