Search Engine Journal points out that MSN Messenger is serving text ads in the chat window and that those ads may be sensitive to the context of your conversation. So I pulled out my laptop and after talking for a few minutes to myself over two different computers, here's what I found. The ads do appear to be context sensitive. I figured if there are ads for anything, that's mortgages, so I pumped some "refinance", "money" and "mortgage" words into my chat. The text ad eventually switched from a generic promo for MSN's own wares (MSN video conversation) to the one you see above - "Need cash? Use the equity in your home..." (click the image to zoom in). The ads were different on two ends of the conversation - on the other client with a newer account and no demographic info, I got a link to download emoticons from Blue Mountain.
This may be different in MSN Messenger 8 Beta as someone at the Search Engine Watch forum pointed out. I used the 7.5 version; have to wait for the 8 to see for myself and there are no relevant screenshots on the web yet.
UPDATE (26 OCT 05): "An MSN executive is disputing that MSN Messenger is running contextual ads based on the users' instant messaging conversations, as recent reports suggest. MSN Messenger is, however, serving up text ads based on its users' age, gender, and other demographic information. -- DMNews