Do you really think that Microsoft invested $1 Billion in Nokia without having some protection against another company swooping in and scooping up Nokia, thereby benefitting from Microsoft's generosity?
I think that at the very least, Microsoft has a "right of first refusal" clause in that investment contract. And if so, who is going to bid for Nokia when there is no real chance they can succeed? The only way would be to overbid so terribly that Microsoft say, "Take them at that price." So you either make a bad deal or no deal at all.
All companies are acquireable. There might be some clause such as MSFT getting some kind reimbursements in the case of acquisition. But it will not prevent the bid from happenning. Of course, the best if for MSFT to acquire NOK so MICROSOFT would have equal footing against APPL and GOOGLE, both of them making phones by themselves