resolved the problem. sure, the windows firewall was the first thing to take down :D but that didn't help. so I dug, dug and dug. Changed settings, ran/stopped services. at one point that special PC lost all the other PCs. In the end it turned out that I had to delete a registry DWORD value which sets the type of your node connection (ipconfig /all in cmd to view) - now everything rolls super fine. Maybe only the deletion of that value isn't the thing (maybe it's the combination - enabling some services, enabling the local net user guest etc) that put everything to where it should be, but it works.