Password protecting in Windows 10 networking

[#1]

Accessing another PC on home network :

C:\Users\xyzzy>net use y: \\192.168.1.111\c

Enter the user name for '192.168.1.111': admin

Enter the password for 192.168.1.111:

System error 86 has occurred.
The specified network password is not correct.

My account name & password does not exist on remote PC

[#2]

A user login password isn't going to protect the machine from a network based attack.

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[#3]

@Mook  Define attack, this is a simple request to deny access to two computers from a third.

[#4]

Fair enough, the lodger may not have nefarious intent but just because something has a password does not make it secure.

[#5]

@Mook  I never said a password was absolute security, it was a non complicated, or expensive, solution to a simple request as above.  Risk assessment of the Lodger is down to the OP.

[#6]

The horse may have already bolted in the interim anyway!  

[#7]

Apologies for not having replied on this topic.  I have been away for two weeks and am now back trying to catch up.

So as suggested, I tried to password protect the "Upstairs" and "Downstairs" computers but that did not work unfortunately.  The other computer on the network never had to enter a password at all which is very odd but had full access.  All it did was ironically mean that I had to enter the password on the main ("Downstairs") computer whereas I did not have to do so before.  So it just inconvenienced me but did nothing to stop the "Upstairs" computer from accessing the "Downstairs" computer which was extremely odd !

So I am now back at square one.