On *your* system it didn't uninstall. It is supposed to uninstall.
Since it didn't, you should manually remove Defender from the system
if you are running Microsoft Security Essentials.
You are correct, MSE does not include Software Explorer,
unfortunately, as was the case with OneCare before it. To manage
startup items you will need a 3rd party tool or simply use MSCONFIG to
disable startup items as desired.
Optionally, you can leave Defender installed on your XP machine, but
be sure to disable it. Not disabling it is a bad idea as the two
programs *will* conflict, causing performance problems and even system
instability.
As for my original response, it is far from "rubbish" and "crap."
I have been supporting MSE in the MSE forums, working closely with the
MSE team and support organization since it was first released to beta
last year. I know of what I speak. And, yes, I have installed it and
am running it on many systems. There have been cases reported where
MSE failed to remove/disable Defender - that's a bug that hasn't yet
been fully resolved. It is basically an installation failure due to
something unusual about the affected systems such as yours.
-steve