Final update: Hugh really spent more than two hour of his time to work, via Skype and Teamviewer, on this problem with me. The graphic problem remained unsolved, but he could point me to the solution of the router problem that I also had and afterwards I couldnt only join, but even, after a very long time, open rooms in VP. Very oddly, for whatever change that happened in the process of trying to solve the graphic problem, I could, afterwards, get my graphic-card's driver settings to override the settings of VP, so everything is resolved now: I get full window with anti-alias. Phew...
Biggest thanks to Hugh: putting two hours of your spare time on helping a total stranger solve a software problem is something. Oh, and according to the deal: He can call me a numpty for the next five years. ;)