Currently avast doesn't scan web based (http) email, I don't know if that is likely to change in the near future. It is not using the pop3 protocol and as such is not scanned by the Mail scanner.
The thread below can provide more info, you can also use the search button at the top of every page and enter 'hotmail' without the quotes and this will bring up much more.
http://forum.avast.com/index.php?board=2;action=display;threadid=4463;start=msg32623#msg32623I use another program MailWasher pro to filter spam and any other unwanted email on my hotmail account. The beauty of this is that it examines the email at source (on the hotmail server) without downloading it. You can then delete those you have selected from the server and download the rest using your email program.
You could also forward your hotmail to one of your pop3 email accounts and then these would be scanned by the normal mail scanner.
Unfortunate but that's the reality.
HTH David