Due to Fitzcarraldo's solution, I've added the location to check mark the PAE box.
You'll open it, when you select your Virtual Machine (left side) in VBox Main window,
then you'll see 'General ... System ... and so on in right window for that selected machine.
For 'System', you'll see the following:
Then click on 'System' headline(right side), and you'll get a new window:
On bottom of it, you can mark PAE now, confirm with 'ok'... That's all.
I don't expect a problem though my windows are in German language ...
-Linuxfluesterer (I love KDE ...)