I had same problem and spent a day to install April CTP.
And, yesterday, I can finally download the CTP release which is installed correctly.
The problematic install file can be identified easily by looking at the publisher field of "Security Warning" dialog which pops up when I run the MSI on Win 2003 SP1.
The publisher field shown on the Security Warning dailog was "Unknown Publisher".
For the working version, it should be "Microsoft Corporation".
Hope this help.