You can download version 5.0.3805 from here: http://www.mvps.org/marksxp/WindowsXP/java.php
You can download an upgrade to version 5.0.3810 from here:
This, however, does not include the 2007 DST change.
That is a version *higher* than 5.0.3812.
(FYI: New Zealand uses 5.0.3815 as per http://blogs.msdn.com/nzdst/archive/2007/10/23/nzdst-microsoft-jvm-version-3815-release.aspx)
To get that DST-updated version, make sure you have $4000 and go to the "Daylight Saving Time changes in 2007" section at http://www.microsoft.com/mscorp/java/default.mspx
The version number is version of C:\windows\system32\msjava.dll
You can uninstall it with a MS-created tool but MS has pulled it because you can only uninstall once and then you cannot re-install it afterward (I haven't tried)
You can uninstall manually as per Java.com (http://www.java.com/en/download/help/uninstall_msvm.xml)
The best tool to test your java installation on you browser of any kind any OS is: http://www.javatester.org/version.html