Do you mean that you want to use MSN as your web browser? Do you use hotmail as your e-mail client? This would give you the option of using Hotmail and Messenger.
The bad news is that MSN explorer isn't available with Windows 7. There appear to be some registry edits that may work and there are suggestions that it may work in Vista compatability mode. I must advise caution here.
If you are using Firefox and you want MSN to come up as your home page, with a browser, then you can change this setting but the whole web page will come up not just the search box.
You can do this by going into Firefox, Tools, Options, Start up, then paste in the URL of MSN into Home Page, click on use current page and te ok at the bottom.