Nothing special about Windows 10. Microsoft have been able to do this for decades. And you sign your rights away simply by activating the software, which by definition indicates your acceptance of their EULA and its "privacy" statement.

This is from the Windows 7 version.

"Microsoft may access or disclose information about you, including the content of your communications, in order to: (a) comply with the law or respond to lawful requests or legal process; (b) protect the rights or property of Microsoft or our customers, including the enforcement of our agreements or policies governing your use of the software; or (c) act on a good faith belief that such access or disclosure is necessary to protect the personal safety of Microsoft employees, customers, or the public."

(I just love the use of "good faith". Imagine what it would be like challenging their definition of "a good faith belief" in court. You would need tens of millions of dollars just to pay your lawyers for the pre-trial hearings.)

Two things you can do about it. The first is to find an alternative to installing Windows in the first place. Second is to find a team of talented and knowledgeable hackers to advise you on the latest techniques to avoid detection and surveillance. However, please be aware that in doing so you will be placed in the same category as terrorist organizations, who - necessarily - operate in this fashion. So don't be surprised if you get that three o'clock door knock.

Alternatively, you can accept the fact that yes, it is possible that someone might be looking over your shoulder each time you sit at your computer. And be comforted only by the fact that they would need to be pretty persistent in order to find anything remotely interesting.

A question:

Does the Microsoft EULA or privacy-statement allow them to spy on other computers in your local network?

Would it be safe to keep Microsoft only for those things it can do uniquely (such as E-tax and certain games) while keeping most work on Unix/Linux?

See my response to Bazz, Yuyutsu.
You can run a virtual machine (there are a few to choose from, I use Virtual Box) inside of linux which allows you to run windows inside of linux just as you would run any linux program. Once you start the virtual machine, you have a fully functional windows install running which can then run any windows program.

The beauty of this setup is, you can run both systems concurrently but both systems are independent of each other, not effecting each other if one, predominantly windows gets a virus or Trojan or something. Also your windows running in the virtual machine does not have access to your personal files which reside on the linux side of things, unless of course you give it access.

Well Yuyutsu, it is not an easy question to answer.
I think once they are into your machine they can do everything that you would do
such as download files from other networked computers.
Etax can be run as Rawmustard said under a Linux program called wine.
The ATO states that on their etax page so they must have tried it.
I only use the virus, malware and firewall protection that comes with Debian Linux.
I do have a scanner program that runs once a day and I have never had a virus etc
since I started with Linux about 8 years ago.
The main reason for this is that the operating system area and other users areas are password protected.
So even if you leave your password lying around a villain can only get at your area.
There are Linux viruses but I have never had one.

You get the normal office suites, word processors, spreadsheets, presentation, etc etc
and they cost nothing. Firefox for browsing and Thunderbird is for email.
There are hundreds of other programs that can be downloaded from the Debian software sites.
Other varieties of Linux are very popular such as Unbuntu and Mint etc etc.
I have tried both those and they are good, but I am used to Debian so I stuck with it.
You can download the disk 1 burn it onto a CD and run it without affecting your windows, a suck it and see operation.
The Linux magazines usually have such discs on their covers.

Linux safe Guess again.

https://wikileaks.org/spyfiles4/

Today, 15 September 2014, WikiLeaks releases previously unseen copies of weaponised German surveillance malware used by intelligence agencies around the world to spy on journalists, political dissidents and others.

FinFisher (formerly part of the UK based Gamma Group International until late 2013) is a German company that produces and sells computer intrusion systems, software exploits and remote monitoring systems that are capable of intercepting communications and data from OS X, Windows and Linux computers as well as Android, iOS, BlackBerry, Symbian and Windows Mobile devices. FinFisher first came to public attention in December 2011 when WikiLeaks published documents detailing their products and business in the first SpyFiles release.

Since the first SpyFiles release, researchers published reports that identified the presence of FinFisher products in countries around the world and documented its use against journalists, activists and political dissidents.

Julian Assange, WikiLeaks Editor in Chief said: "FinFisher continues to operate brazenly from Germany selling weaponised surveillance malware to some of the most abusive regimes in the world. The Merkel government pretends to be concerned about privacy, but its actions speak otherwise. Why does the Merkel government continue to protect FinFisher? This full data release will help the technical community build tools to protect people from FinFisher including by tracking down its command and control centers."

FinFisher Relay and FinSpy Proxy are the components of the FinFisher suite responsible for collecting the data acquired from the infected victims and delivering it to their controllers. It is commonly deployed by FinFisher's customers in strategic points around the world to route the collected data through an anonymizing chain, in order to disguise the identity of its operators and the real location of the final storage, which is instead operated by the FinSpy Master.

Yuyutsu, - The EULA is irrelevant, the NSA or police, ASIO whoever do not care the US Government (you can bet they all do it) have been using the telecommunication providers to spy on people for years they even had to go to the extraordinary step of indemnifying them from any legal action made by citizens.

The NSA even went to the extreme of intercepting computers bought by people and organizations and installing bugs in them before delivery.