user129557: There are billions of computers out there running Windows O.S., putting aside the reputation risks, technically, Microsoft could add a backdoor to force users hardware to mine BTC.
Is the combined computer power of those billions of PCs running Windows enough to conduct a 51% attack against bitcoin network?

user13866: Microsoft's software runs on a very large number of PCs. However the computers in question generally don't have specialised hashing hardware - they only have typical central processing units (CPUs) that are far far slower at hashing than the ASICs used by miners. A million desktop PCs might have less hashing power than a single ASIC.
See Is the native Bitcoin client intended for mining?

According to Ycharts, the current bitcoin network hashrate is 202.17M TH/s for Dec 27 2022. That is 2 x 102 x 106 x 1012 = 2 x 1020
An Intel® Core™ i7-2600 / 8 / 3.40 GHz has a hashrate of 24 MH/s which is 2.4 x 107
So you probably need to subvert more than 1013 desktop PCs.
Worldometer reports

According to a report by Forrester Research, there were over one billion PCs in use worldwide by the end of 2008, and over 2 billion by the year 2015.

That's only 2 x 109 - If my sums are right, flat out they'd have no chance of making any noticeable impact whatsoever on Bitcoin hashrate.

129557: <p>There are billions of computers out there running Windows O.S., putting aside the reputation risks, technically, Microsoft could add a backdoor to force users hardware to mine BTC.</p>
<p>Is the combined computer power of those billions of PCs running Windows enough to conduct a 51% attack against bitcoin network?</p>
 13866: <p>Microsoft's software runs on a very large number of PCs. However the computers in question generally don't have specialised hashing hardware - they only have typical central processing units (CPUs) that are far far slower at hashing than the ASICs used by miners. A million desktop PCs might have less hashing power than a single ASIC.</p>
<p>See <a href="https://bitcoin.stackexchange.com/q/99248/13866">Is the native Bitcoin client intended for mining?</a></p>
<hr />
<p>According to <a href="https://ycharts.com/indicators/bitcoin_network_hash_rate" rel="nofollow noreferrer">Ycharts</a>, the current bitcoin network hashrate is 202.17M TH/s for Dec 27 2022. That is 2 x 10<sup>2</sup> x 10<sup>6</sup> x 10<sup>12</sup> = 2 x 10<sup>20</sup></p>
<p>An Intel® Core™ i7-2600 / 8 / 3.40 GHz has a hashrate of 24 MH/s which is 2.4 x 10<sup>7</sup></p>
<p>So you probably need to subvert more than 10<sup>13</sup> desktop PCs.</p>
<p><a href="https://www.worldometers.info/computers/" rel="nofollow noreferrer">Worldometer</a> reports</p>
<blockquote>
<p>According to a report by Forrester Research, there were over one billion PCs in use worldwide by the end of 2008, and over 2 billion by the year 2015.</p>
</blockquote>
<p>That's only 2 x 10<sup>9</sup> - If my sums are right, flat out they'd have no chance of making any noticeable impact whatsoever on Bitcoin hashrate.</p>
