Well, the thing is that I've seen many 'viruses' and 'trojans' written in both C and C++, which basically annoy you and scare you into doing something stupid, like formatting your C: drive.
The reason you don't see alot, is because it takes some work, which most of these makers of 'trojans' and 'viruses' don't want to put out. (btw, if anyone has found the source code to a real virus or trojan in C/C++, I'd like to know.) What you should do if you really want to make effective viruses is to have a certain target in mind (Windows XP, Dell hardware, etc) and learn some assembler for that particular target, then make viruses.
