I have both JDK 1.4.2_08 and 1.5.0_01 installed on my system. I also note that in Program Files I have JRE 1.4.2_08 and JRE 1.5.0, _01, and _02 installed. The JREs I assume are the result of installing the
Java Plugin while the first two I mentioned are from installing the full JDKs.
So, first make note of exactly what you're trying to install: JDK or JRE. Then, double-check where they're being installed. The error message makes it sound like you already have JRE 1.5.0_02 installed, and it's refusing to write over it.
Looking further, all of these show up in Add/Remove Programs control panel but under differing naming schemes. The 1.5 (aka 5.0) ones show up as "J2SE Development Kit 5.0" and "J2SE Runtime Environment 5.0" while the 1.4 ones are "Java 2 SDK, SE v1.4.2_08" and "Java 2 Runtime Environment, SE v1.4.2_08".
To verify, you can execute "javac -version" and "java -version" with a fully-specified path to the javac/java executable.
Strangely enough, the updates (1 and 2) seem to be full installations and not updates at all. I'm not so daring as to try uninstalling the prior versions, however.