When I dropped my MSN Dial-up account I also began to receive the advertisements with web based Hotmail.
I discovered there is a premium (paid) option available that removes these advertisements and also includes other features like larger attachment sizes (20MB) among others. Since this only cost $20/year I chose this option since I prefer to use a web client that requires no installation.
Here is the US link to the page describing the features for Windows Live HotMail Plus and including a link to purchase it online.
http://www.gowindowslive.com/hotmailplus/en-us/default.aspxRob