Might seem daft, if you download hotmail's IM service is it specific to that PC or will it travel no matter which PC you are on [with your hotmail account for example]
You can use your username/ password to access the IM service from any PC or Mac that has the IM software downloaded to it. (If you're off on your travels, you'll find that an increasing number of cyber cafes have IM installed: very handy!)