I'm not familiar with all the Norton products. If your Norton product is a
suite which includes anti-spyware functionality, I would expect it to
automatically disable Windows Defender on installation.
Microsoft recommends that only a single product doing real-time scanning be
installed, if possible.
Microsoft Security Essentials will request uninstallation of other
anti-virus products as part of the installation routine, and it will disable
Windows Defender, because it incorporated Windows Defender's functionality.
A better way to phrase your second question would be "If I am using
Microsoft Security Essentials, should I also use Windows Defender?" and the
answer would be no--Microsoft Security Essentials will disable Windows
Defender.
In terms of your final question, I'm not Microsoft Security Essentials
definitely incorporates Windows Defender's protections, so they are
redundant, but I would always let the Malicious Software Removal tool run as
part of the monthly security updates--it is possible that there is
redundancy there, but this is a once a month run--I wouldn't bother to take
any action to change the way things work.
So--in summary, I would recommend that if you've purchased a security suite
which provides both anti-virus and anti-spyware, that you allow it to
disable Windows Defender, and trust the vendor you've chosen to protect you.
If you want to switch vendors, Microsoft Security Essentials provides
well-reviewed effective coverage at an excellent cost for home users. The
Malicious Software Removal tool may be redundant, but I wouldn't worry about
it.
