I downloaded and attempted to install December 2004 CTP of SQL Express.

http://www.microsoft.com/downloads/details.aspx?amp;displaylang=en&familyid=66120BE8-826A-482A-8264-B1836F28FD79&displaylang=en

However, on running installation, I got an error along the lines that I had old versions of the .NET Framework 2.0 installed. I checked Add/Remove Programs and confirmed that this wasn't the case. After a big of digging, I looked in the "C:\WINDOWS\Microsoft.NET\Framework" directory and discovered that a file containing version 2.0.40607.42 (aka Beta 1) still existed on the machine. Deleting this folder allowed me to install Sql Express

Version 40607.42 can be found here

http://www.microsoft.com/downloads/details.aspx?FamilyID=B7ADC595-717C-4EF7-817B-BDEFD6947019&displaylang=en

Version 40903.18 can be found here

http://go.microsoft.com/fwlink/?LinkId=37074

Version 40903.27 can be found here

http://download.microsoft.com/download/9/A/4/9A47D72B-9D98-46CC-87F3-B92E97888E4
B/dotnetfx.exe

I believe that both .18 and .27 will work for December 2004 CTP of SQL Express but stand to be corrected

However, all this is now old news, as the February 2005 CTP is out

http://www.microsoft.com/downloads/details.aspx?FamilyId=08215D3F-9A1D-483F-8E21-A2EE19936899&displaylang=en