Install/unistall steps:
1) install JRE 1.3.1_15;
2) install mustang (I used b46 to reproduce the problem)
3) uninstall mustang

There were extra keys created under HKCR and HKLM as follows:

[HKEY_CLASSES_ROOT\CLSID\{CAFEEFAC-0013-0001-0015-ABCDEFFEDCBC}]
@="Java Plug-in 1.3.1_15"

[HKEY_CLASSES_ROOT\CLSID\{CAFEEFAC-0013-0001-0015-ABCDEFFEDCBC}\InprocServer32]

@="C:\\Program Files\\JavaSoft\\JRE\\1.3.1_15\\bin\\npjava131_15.dll"
"ThreadingModel"="Apartment"

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\CLSID\{CAFEEFAC-0013-0001-0015-ABCDEFFEDCBC}]

@="Java Plug-in 1.3.1_15"

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\CLSID\{CAFEEFAC-0013-0001-0015-ABCDEFFEDCBC}\InprocServer32]

@="C:\\Program Files\\JavaSoft\\JRE\\1.3.1_15\\bin\\npjava131_15.dll"
"ThreadingModel"="Apartment"

The clsid ending with "BC" is not supported in 1.3.1; we started
supporting that clsid in tiger.