Like many Vista testers, one of the first things I did after installing Vista on a test PC (a mandatory clean install in the early beta days) was to install a bunch of must-have free applications: Macromedia/Adobe Flash, Adobe Acrobat, Firefox, Thunderbird, and a bunch of other apps. Although Java isn't an app, there are a number of apps that require the Java Runtime to work. Unfortunately, it looked to me like JRE/JDK5 simply wouldn't install. I didn't get it. Fortunately, Chet Haase does and explains the Vista/Java relationship in his blog:

Java on Vista: Yes, it Works

It turns out that the trick is to download and install Java SE 6 (currently in beta) found at:

https://jdk6.dev.java.net/