Since 1.5 I have the problem that my character cannot activate the Guardian Stones anymore, when I try to start a new game. Hadvar just runs past them after Helgen instead of stopping and explaining them, after which one can normally choose one effect.

I did a complete reinstall with no mods loaded, nevertheless Hadvar ignores the stones.

Is there a way to activate them with a console command? They have a script attached "Powershrinescript". I looked up the various commands for the console, but couldn′t find one that would do this. I remember from Morrowind and Oblivion that is was possible to start a script ingame.

Or is there another way to get them activated?

Check UESP for console commands....just started a new game last week and he did his normal show and tell on the way to Riverwood.

I also prefer to not choose a guardian stone at all to level a little slower on Master because I love to torture myself.