MSN Premium is 9.95 per month in the US (plus tax).
It gives you the use of MSN Premium, Webroot Spysweeper, and McAfee antivirus. it removes ads from Hotmail and Messenger. It gives you increased storage and sending limits for Hotmail.
MSN Internet Access gives you dial-up access and I think that's 19.95 per month.
Windows Live Hotmail Plus is available for about $20 a year in the US which only gives you the increased storage and sending limits, plus removal of ads. No use of MSN Premium.
You can cancel any of the paid services and retain your @msn address as a free account (with ads) that can be accessed at http://www.hotmail.com or using any number of email programs:
How to access Hotmail with various email programs:
http://windowslivehelp.com/solutions.aspx?categoryid=3bb8e263-8810-483f-9865-520317751005
-steve