Network Password?

Every time I try to figure this out, I spent half an hour or more Googling, reading posts people have made - and give up, defeated. Maybe someone will be able to help me resolve this? (I think I posted here a long while ago on this subject, but cannot find the thread: it didn't sort things out in any case).

I have two PCs and a laptop linked on my home network. One PC ('PC 2') and the laptop see other each both ways without any password window popping up: I can share files.

It's the other PC ('PC 1') which presents the problem: it can be seen by the PC 2 and the laptop, and I can transfer files that way; but if I try to access PC 2 from PC 1, I get a password box up, as below. If I access the laptop from PC 1 though, no password box comes up and again, I can access folders and files there. It's just PC 1 to PC 2 that doesn't work. I don't know which of the PCs is causing the 'blockage', either.

All three are on the same workgroup name. Problem also is, I have no idea what I am supposed to enter as the password and user name... lots of tutorials on the net saying you should look at this or that tab on Properties box or whatever, but those tabs or options don't seem to be there when I try (which I why I have always given up).

The two PCs multiboot: one between Windows 7 and Windows 10, the other Windows 7 and Windows XP. The laptop only has Windows 10. But the issue isn't with the different versions of Windows: the PC in question, PC 1, on Win7 or XP, links to the laptop (Win10) just fine, but it won't open folders on the PC 2, whether the latter is booted into Win7 or Win10.

All folders have correct sharing options set, with full permissions, and set for 'Everyone'.Also all computers set not to use passworded connections in the network options..

Maybe if I knew what to fill in in the window that appears...

Thanks for any help with this. Don't know what else to try.

Martin

It will need the username and password of a user on that pc its a bit strange as it says enter password for martin-fspc but gives domain martin-pc. Windows doesnt like names with - in them and when you connect it by default passes username password your logged in with and the share permissions must be right and also NTFS permission

It will need the username and password of a user on that pc

'that PC' means PC 1 or PC 2?

Windows doesnt like names with - in them

.. could be, but PC 2 to PC 1 works fine, whereas the reverse doesn't, which doesn't make sense to me, at least as far as PC names are concerned.
PC 2 has a PIN for access.. would I have had to set up a password too? I don't recall now. I am far more comfortable 'under the bonnet/hood' in Win7 than Win10, I should add; only fairly recently given in and am slowly going over to Win10 (though PC 1, with the 'issue', will always be Win7/XP)..
All partitions are NTFS.
Thanks.

EDIT: Actually, there is one other oddity: in fact, when I try to access PC1 from PC 2, a box comes up saying it can't connect, with the option to diagnose the issue. If I click diagnose, the connection instantly becomes active, even if I close the diagnose box the second it starts. If I let the diagnostic run to the end, it says no solution was found. But each and every time starting that diagnosis instantly enables the connection. Does that make any sense at all?