[...]

I used to have this problem. Now I pass the following on the startup command
in my shortcut to launch Eclipse:

-vm C:\j2sdk1.4.2_04\jre\bin\javaw

Of course, you should change the path depending on your installation of the
SDK. If the JRE thats used upon launching Eclipse doesn't contain the
javadoc, you won't see it. This happens usually because the standard
installation of a JRE is different from the SDK -- Eclipse sometimes finds
the wrong one. In my case that's due to the numerous installations and
upgrades, etc. Multiple JREs are often installed on a system. For example
Oracle installs a JRE. By being explicit on the launch, you take away all
doubt. Hope this helps!

Also, I specify my project path (which is outside of the Eclipse install)
with the -data option (in the same Eclipse launch shortcut). This helps
because you don't have to copy your data files EACH time there's a new
release. Helps for backing up the right files, etc. Indeed -vm and -data are
very useful items.