By default, when the Private computer checkbox is visible, it is checked by default, meaning that OWA will store some things in your browser by default. Sometimes, this may not be desirable, and you may prefer the checkbox is unchecked by default so as to prevent this. There is no setting to change this default, but there's a workaround, allowing you to obtain the same result. The only downside of this workaround is that it needs to be done anew after each update of Exchange.

To show the Private Computer checkbox in the first place, read this article about enabling the option.

To apply the workaround, you need to manually edit the logon.aspx page so it does not check the checkbox by default (which is the reason why it gets reset with every update to Exchange).

Warning

Be sure to create a backup of the logon.aspx file before changing it. Incorrectly changing the file may break the file and render OWA unusable!

1. On the Exchange CAS server, navigate to the folder where Exchange is located, and subsequently to the subfolder:
   <Exchange-root>\V15\FrontEnd\HttpProxy\owa\auth
2. Open the file logon.aspx with a text editor.
3. Search for the text chkPrvt to locate the <input> tag, which is in fact the checkbox in HTML-code. (If you do not know what I mean with this, you should probably not change the code!)
   It will look somewhat like this:
   <input id="chkPrvt" onclick="clkSec()" name="trusted" value="4" type="checkbox" checked role="checkbox" aria-labelledby="privateLabel"/>
4. Notice the checked keyword in the middle. This keyword needs to be removed so it reads:
   <input id="chkPrvt" onclick="clkSec()" name="trusted" value="4" type="checkbox" role="checkbox" aria-labelledby="privateLabel"/>
5. Save the changes to the file. Clear your browser cache and try again: the checkbox should now be unchecked by default.