I don't think dragon-x wants to be that protective, but yeah like TheSickness has said, MSN 6 and the latest 7 has a option to turn on conversations.
Tools, Options, Messages, and then click on Automatically Kepp a history of my conversations. You can then choose where to install it, like TheSickness has said
It also has no way of storing passwords so she couldn't stop you looking, unless she found this out, and kept turning the option off (this is possible)
If she has MSN Plus though (an add-on to Messenger, it would store all the conversations, but she'd be allowed to place passwords on to protect her) but you could also turn it on for yourself.
I do warn you though, she'll probably have a password to log on and won't automaticaly log on when windows does, and in this case, you'll have to get that, and I don't think that would be possible.
I don't really agree to what you are going to do, and I'm sure shes safe. No one wants people knowing about who they talk to and what about (even if its nothing bad), especially if shes a teenager like me.
I don't get up to anything, but I use MSN Plus to put passwords on, and don't like my parents talking about my mates on it, though they trust me that I don't do anything, so you just have to be wary.
I hope this helps