Just some quick comments: WLM stores your email on their server ánd on your computer when you use hotmail or live mail. and I think any other webbased email (not sure about the last one). It stores the files in C:\Users\*username*\AppData\Local\Microsoft\Windows Live Mail.
When you use WLM for other email (POP3) this does not work and files are only stored on your computer.
So the 'in the cloud computing' only goes for the Microsoft part :)
(to me that was a relief - I don't think in the cloud computing is very wise, even if it might be convenient. See what I wrote about it
What Os did/do you use before 7?
If you have Vista you can take Windows Mail with you. Just copy the folder to an USB stick or something and copy it to Program Files when you have 7 installed. I don't know if this also works for Outlook Express (would sure like to find out ;D).
I remember from my old Win98 that I had a seperate download to reinstall Outlook Express (messed it up one time and had to reinstall - that was still possible with Outlook Express, not with Windows Mail). Google it a bit and see if you can still find that download. Might be worth the while :)
p.s. I also like Outlook Express/Windows Mail best
Edit: found a link here: http://www.tucows.com/preview/205178
Second edit: this link is for OE6, even better:)
Does also install IE btw, but if you don't use that and use another browser that should not be a problem.
Btw - I have no idea if this is going to work on 7, you'll have to give it a try, so make good backups etc. But I take it you were aware of that already :)