If you are still having this problem, here is the solution. I have verified it by syncing between webmail, 2009 version of Windows Live Mail (WLM), and 2011 version of WLM. I did this first using hotmail, and then using a private IMAP email account on one of my domain hosts. It works just as you request, automatically! There are several points, however, at which one might have problems:
1. In WLM, you cannot change a POP3 account (the default) to an IMAP account. You must delete it and start over, and use manual server configuration, and the settings and ports given by your mail host provider. To avoid losing mail when deleting the account, drag messages to a storage folder, and drag them back when finished.
2. The initial sync is very slow. Might take hours to sync a few thousand messages, such as I have.
3. It only syncs folders in the actual IMAP mail accounts. It does not sync "storage folders." There is no way to sync storage folders which are only on your hard drive and not on the IMAP server, because WLM arbitrarily renames the message files.
4. Some IMAP accounts follow a strange hierarchy. ALL folders are within the INBOX folder! This is not true of Hotmail, but it was true of my private email account. I got an error when trying to create outside the INBOX hierarchy.
5. When I created a new folder using WLM on one computer, it automatically appeared in webmail, and so did any messages that I dragged in there. BUT, the folder would NOT appear on the other computer no matter how much syncing I did, even a "sync everything." I had to create the folder manually on the second computer. When I did that, WLM automatically synchronized it, and the messages appeared. So you have to create the folder hierarchy manually on each computer. This actually could be a "feature" if you don't want all the folders to appear on every computer, for privacy or efficiency reasons.
WARNING! While in most cases WLM seems to actually store copies of messages on all computers, in a few cases it had to retrieve the message from the server when I tried to open it. If anyone has more insight to this, please post a reply, thanks.
Since it syncs the full message with attachments, you can use this like a general records archival and sync, without having a separate account for that.
It took several days to figure this out because I was not familiar with IMAP or WLM. But now I am very happy with it, because it eliminates the manual syncing I was doing on Outlook Express between computers, which I would occasionally mess up. Hope the info is helpful.