What is Windows built on? Is Windows still built upon the MS-DOS
'kernel'? Or is it not anymore? Because I know that MS-DOS has stopped
being developped since 2000. Will Longhorn be built on DOS, or will it
be built on a new kernel, or what?

Was Windows built entirely from the ground up by Microsoft, besides
the original DOS that Bill Gates purchased from another software
company?

Also, the Mac OS X is built on UNIX, right? Did they license the UNIX
from the original company that owns UNIX?

Windows XP is in fact a version of Windows NT and is based on a
Unix-like kernel developed by Microsoft. Mac-OsX has an opensource
kernel (mainly it can be used distributed and modified by anyone see:
http://www.opensource.org/). It is essentially based on freeBSD
(http://www.freebsd.org/) which can be used and modified by everyone
without license. The graphical interface of Mac-OSX is "proprietary"
(i.e. copyrighted) and developed by Apple. There are indeed several
"Unix" kernels available ("Unix" is a trademark refering to such a
particular kernel): FreeBSD is one, Linux is maybe the most known one
(linux is just the kernel; on which we add utilities; graphical
interface). Due to the fact that both MacOsX and system based on Linux
use both a Unix kernel; it is usually possible to recompile a software
destinated for Linux on MAcOsX.

But we can only recompile Linux software for the Mac, IF we have the
source code, correct?

Also, when you say UNIX-like (regarding Windows) what similarities are
there, then, between the MS version and the real UNIX?

Also, why is UNIX considered super, extremely stable, while Microsoft
has shabby design and coding?

Yes we can only recompile Linux software on MacOsX if we have the
source code. But most programs available on Linux are available with
their source code so this is generally not a problem. Note also that
it is also usually possible to recompile Linux software on Windows by
using http://www.cygwin.com (which emulate a full Unix "Posix"e system
on Windows). Wine (http://www.winehq.com http://www.codeweavers.com/
allow to run some of windows programs (most notably word,  powerpoint,
etc...) on Unix/Linux system based on an intel computer; even in
binary form when no source code are available. Unfortunately, wine is
still very unstable.

I do not know much about the Windows kernel and moreover the source
code is not available. I just know that the kernel is Unix-like but am
not sure it is really visibel from a user point of view.

For the stability; it was essentially wi 9x which were considered
unstable. Windows NT and XP is much more stable. The fact is that
Microsoft has always privilegied the ease of use than the stability. A
real problem today regarding Windows is its vulnerably to virus. It is
due partially to the fact that Windows is the most used operating
system but also to the fact that internet explorer and outlook express
can execute programs too easily: there is no real difference between
executing opening a file (such as an image) and executing a program.

Okay, thanks for the comments.

Another thing:

If Unix and/or Mac OS X became the most popular operating system (just
bear with me here) then would it have the same amount of viruses as
Windows has right now? Personally, I seriously doubt it, since Mac OS
X and Linux, etc. are built on a much more  secure foundation than
Windows is.

Please share your thoughts on this.

This is not easy to answer. 

I don't believe the vulnerability of Windows to virus is due to the
kernel but mostly from IE explorer and outlook express which allow too
easily to execute programs. It is known that this is safer to use
alternate softwares for these pupose such as Firefox and thunderbid
(which are very  common on Linux). There is also the fact that all is
done to encourage the novice user to remains "administartor" which is
also dangerous. Another related problem is the graphical interface:
you just click on a program to execute it (which is potentially
harmfull) or to open it (which should be safe) without really seing
the difference. Given the fact that the extension of the files are
hidden by defaultsand that the icon of an executable can be freely
choosen, it is easy to make an executable virus with the icon of a pdf
file...
Altrough I do not know MacOsX very much (I mainly use Linux and
sometime Windows XP); I've heard that it share at least some of the
same problems.

Linux is usually safer: you are not encouraged to stay "root"
(administrator). In order to execute a file you must first explicitly
make the file executable which suppose you know what you are doing.
The problem is that for a novice user which does not even know what
executing or opening a file is, it might be a little more complicated
tan just clicking. Also the browsers or mail readers on Linux are
usually much safer.