If you install JDK 6u4-b10 on WindowsXP and you choose not to install the "Demos and Samples", you will get several error messages and then you see a window saying the "Installation Failed". I also tested this using 6u3-b05 and this did not happen. Please see images attached.
After verifying the issue, I found that ...

1. Installation will fail, whether the user de-select the "Source code" option, if the user de-select the "demo and sample" option.

2. After failing the installation, the installation dialog box would roll back the OS to the original state. No JDK entry would be in the Windows registry, but it would be in the Windows directory.

3. It happens on both XP and Vista.
the regression is introduced in build 6u4b3. does not exist in 6u4b2 (formerly 6u3b2).
see bundle naming:
http://jre.sfbay/java/re/jdk/6_04/promoted/all/b02/bundles/windows-i586/
http://jre.sfbay/java/re/jdk/6_04/promoted/all/b03/bundles/windows-i586/
User now cannot select the options.