No. Do your research. Java can be faster than MSVC++.
I agree there. It's not because you go from being on one platform to cross-platform using a java-like structure, that this automatically means things will get slower.
It depends, I think, on how well those cross-platform antivirusses will be coded.
And I think that MS has had a long and succesful journey, security-wise I mean. If I remember correctly, Windows ME was a disaster, security-wise. XP was better, Vista even more and now that we have Windows 7... Well, let's just say: the security is not perfect, and it probably never will be, but I myself congratulate MS on their progress towards security. It aint perfect yet, but it's already a lot better.
Mac on the other hand, well... they haven't really had much reason to work on their security.