Let's try to cover the main issues here.
You seem mostly interested in your Hotmail account. Hotmail is not your common or garden email account.
Let's just touch on non-Hotmail accounts before coming back to Hotmail.
For regular email accounts that provide you with POP/SMTP servers there are two possibilities:
1) you are allowed by the mail servers to use unsecured ports (typically 110 for POP and 25 for SMTP). Then avast can scan these emails (and insert "clean" messages id you wish)
2) you are (more and more frequently these days) required by the mail servers to use secure connections (more for their benefit than yours to be honest) but this means that the present release of avast cannot scan the emails delivered this way. They can be but it requires that you configure and run a free third party program called STunnel that manages the secure connections and still allows avast to scan the emails safely and securely within your own system.
So let's get back to Hotmail.
Microsoft wants you to use Windows Live Mail (WLM) with your Hotmail account. With WLM you get, with any Hotmail account, an IMAP like experience where it synchronizes all all your folders on the server with WLM. As you have been advised it uses a proprietary Microsoft protocol called DeltaSync. This is, as David described, the secret successor to the old WebDav protocol that manages "http" enabled Hotmail accounts that work with Outlook Express (and despite the comment by David is, in fact, still alive and kicking despite being under sentence of demise as some at yet to be specified date). The reason MS makes all Hotmail accounts work with WLM and DeltaSync is, surprise surprise, because this is a much less expensive load on their Hotmail server farm. As you have been advised avast does not now and cannot in the future scan emails delivered in WLM via the DeltaSync protocol.
At the same time the Microsoft folks don't want too much trouble over their proprietary access method and all those nasty antitrust objections. So after they rolled out WLM and got as many folks as possible to use WLM and Hotmail they then also rolled out regular POP/SMTP servers for Hotmail (again - after a gap of many years). So you can now use POP/SMTP servers in many countries (the UK for one) to access your Hotmail account with any email client (including WLM). However, they do require that you use secured connections to access their POP/SMTP servers (please see case 2 above for POP/SMTP servers). So, to scan Hotmail now you would need to use Hotmail's POP/SMTP servers and use the STunnel program mentioned above.
avast is planning to deliver this year release 5. Within release 5 they have advised the function of STunnel to manage secure email connections and allow avast to scan the emails delivered on them will be provided within avast itself. That means if you can wait a little longer and you use the Hotmail POP/SMTP servers for your Hotmail account in WLM (rather than the "http" connection) then avast will be able to scan the Hotmail messages.
Other options:
There are functions that convert Hotmail accessed via the Web interface to POP and even some that will provide SMTP conversion for Hotmail.
I will not go into these in detail unless you want to after reading the above information. Suffice to say that I get my Hotmail delivered daily in Thunderbird via the Thunderbird Webmail add-on that allows me to read my Hotmail accounts, send to my Hotmail accounts and have them scanned by avast, including downloads from custom folders on Hotmail too. Oh, they do the same for my Yahoo accounts as well.
