Windows 10 Activated

Getting an activated version of Windows(r)  is very simple particularly Windows(r) 10 since you can get a copy from their website and all you need to do is deploy KMS - key management service, which again is explained in their website.

To download Windows(r) 10, head over to Windows site at this link
Windows(r) 10 media creation tool

To deploy KMS, Microsoft explains in this site
Deploy KMS

To avoid all that hustle, you can get a copy of build 10240 here

Xamarin Offline Installation

Installing Xamarin can be really painful using the installer available from their website. A better option is to download the installer from here then downloading the components one by one, install all of them and then running the installer so that it can configure the components.
As you can see from the image below the installer does the downloading and installation at the same time which makes it slow.

Using a download manager like IDM you can lessen the time you require to install Xamarin.

Download Links
1. Java
Jdk v7u71

2. Android SDK
Android SDK

3. Android NDK
Andk x86
Andk x64

4. GIMP Toolkit
GTK# v2.12.26

5. Xamarin Studio
MonoDevelop v5.9.5.9-0

6. Xamarin Visual Studio
Xamarin VS v3.11.836