You are probably getting a ""The webpage cannot be found error." when trying to view the Search Administration page.
The reason for this error is that it is at the moment not advisable (MS speak = "should not") to install Search Server 2008 Express on a server where MOSS is installed.
An update is promised for the first half of 2008 which will "also support upgrade to MOSS 2007"
The above text comes from the posting of a Microsoft support person (SK) in a public forum.
That posting is for me partially unclear so I enclose it below in italics.
(or do you for instance understand "the Microsoft Search Server release is not itself intended to be the mechanism of upgrading MOSS 2007".?)
Symptom: When , you see "The webpage cannot be found error."
Explanation: You may see this if you've installed on a server that also has Microsoft Office SharePoint Server 2007 installed. Search Server 2008 and Microsoft Office SharePoint Server 2007 should not be installed on the same server at the same time.
For customers of MOSS 2007, we will provide a public update containing new features from Search Server in the H1 CY 2008. At release time of Search Server 2008, we will also support upgrade to MOSS 2007 + this Public Update. However, the Microsoft Search Server release is not itself intended to be the mechanism of upgrading MOSS 2007.
Workaround: Install Search Server on a different machine, or first uninstall MOSS 2007.