Is your computer in the living room? I think that its best if computers are in a shared space not private so that its easier for you to see what she's up to.
Also with MSN I think that you can set it to save conversations. At her age I don't think its an invasion of privacy to just check her conversations randomly every now and then.
I often told my mother that my freinds were allowed to do this that and the other - it was BS but there was aways the hope that she'd believe it lol. She never did .