Windows issues - I need to "active windows"

I am having some computer issues - damn thing says I need to activate windows. Problem is that it gives me a yes/no alternative and neither options makes anything happen.

It all happened when a friend of mine war borrowing the computer when the battery on the mouse went flat, he bought new batteries and still couldn't get it to work - obviously because he forgot to synchronize the receiver and the sender. So for some odd reason he figured he'd restart the computer and press F10 in the boot screen. What he did next I have no Idea but the computer now refuses to log into my account because I need to "activate windows".

I managed start in that safety mode-thingamajjigger to reset the computer to and earlier time(don't remember what it's called in english) - that didn't work either.

So, now I'm out of ideas. I was really hoping some of you could help me.

Thanks in advance.