URL:
https://adcenter.msn.com/Default.aspx
If you have not got an invitation, you can sign up here:
https://advertising.msn.com/msn-adcenter/signup
It took 2-3 days to get my invitation after i signed up.
So what is MSN Adcenter?, Microsoft's answer to Google ads, but better

.
You get to specify what days you want your ad to run, what hours, you can even specify to what type of visitor you want (MSN has information on certain users, i am guessing all hotmail users, ie. their gender, age, income, etc.). I am pretty amazed at the system so far, however, its still beta, so I don’t think its running on the live MSN site, only on certain areas, so you will not get a huge amount of clicks, however, why not start your account now, before everyone else does

, chances are you’ll pay a lower CPC at least until everyone catches on.