Well, for Windows, there is a registry that get updated with all the pertainant information.
I can browse my NT registry using
C:\WINNT\system32\regedt32.exe
If you look in the Local Machine page under JavaSoft there are entries for JDK, Plug-in, JRE, Java WEb Start and prefs. In those it names the locations for the OS to look.
Normally when you download a Plug-in for a browser the registry gets updated for you during set-up. The Plugin can be downloaded from
here.
Of course if you are using XP - I have not a clue.