Hello,
I have not try your product yet, but I have read the tutorials.

Is possible the following senario?

In Windows 2008 Server, I have a VM at www.master.com.
master.com contains 15000 products in 3 "main" categories.

Both 3 "main" categories contain many sub-categories,
that contain other sub-categories and so go on.

I need to create a slave VM at www.slave.com.

slave.com should be hosted in the same machine as master.com.
I 'd like both sites to share the same mysql db.

slave.com should have different css template/template images.
slave.com should have different joomla and/or VM users.
slave.com should display only one specific main category and
all the related sub-categories and products (using views and NOT tables !!!)

I should NOT add products or categories at slave.com
I should ONLY add products or categories at master.com

When I add a product or category (at master.com),
that it is under the specific main category,
slave.com should display it.

So, is this possible?

Also, because I have a total of 5GB HD space only (2GB free)
and because I read that "the content will be either a copy (for Windows
servers) or a symbolic link (for Unix servers)",
I 'd like to suggest:

en.wikipedia.org/wiki/NTFS_junction_point

Hard Links and Junctions (supported in the NTFS file system):
msdn.microsoft.com/en-us/library/aa365006(VS.85).aspx

Symbolic Links:
(Minimum supported client: Windows Vista
Minimum supported server: Windows Server 2008)
msdn.microsoft.com/en-us/library/aa363878(v=VS.85).aspx

Thanks, in advance

Concerning Symbolic Link, this is PHP related and the functionality is not available for Windows.
This require a PHP 5.3.0 or higher to be available but in our case we do not autorize it.

With Windows Plaftorm, we do NOT repliacte the directories and you have to assign the same root document directory to each domain or subdomain.
You can also use Virtual Directories that pont to the master directory.
See FAQ
www.jms2win.com/faq#sec-112

It is possible to share the VirtueMart between the websites but in this case, you have ALL the categories in the slave sites. Up to you to display only the category that you want with a specific menu but potentially all the categories are available.

If you share VirtueMart, this mean that the same MySQL tables will be used by the different websites and therefore you will be able to update its content from any websites (master or slave).

So YES this is possible to share the VirtueMart directory and you have to share the same master direcrtory.
You will share the same "/templates" directory and will be able to select different joomla template to have different look and therefore speciifc template/images directory.
The "/images" directory will be shared.

When you share VirtueMart, this also share the users. It is NOT possible to have specific users as soon as you decide to share VirtueMarrt as the users are part of VirtueMart (shopper info).