This is a big topic with lots of details and I’m going by my brain’s memory and a few tests, so it may not be 100% accurate with some of the versioning details.
There’s at least three versions of the Messenger objectstore.
The first version from 6.0, isn’t portable that I recall but I don’t remember why.
The second and third versions post 6.0 (maybe 6.1/6.2) are encrypted using the e-mail passport name, and can be moved to any machine but due to the encrypted are limited to that specific e-mail address, with the third version (WLM) also using the Passport ID (PUID).
One solution for non-winks is to painfully re-add them yourself using the .dt2 files. In the past I have copied them into a folder, renamed them all to PNG or GIF (ie. rename *.dt2 *.png) and then used Explorer’s thumbnails to see what each file is. The ones you can’t see can just be dragged to a browser which will figure out the file type and display it. There’s probably other good freeware image viewer applications that can do the same with less effort too.
The other solution involves using the MSN Backup tool by Leonardo Bai. It can decrypt the id2 metadata, then backup the assets into the MCO format, and then those can be reinstalled by using the COM object that the old Emoticon/Wink store used. This does result in anything reinstalled using this method will be marked as premium and the Getoption is grayed out when people right-click on them.
Here is MSN Backup 1.22 and MSN Backup 1.34. 1.34 adds WLM support but unfortunately doesn’t seem to be working now for me as it claims the PUID is wrong, although the PUID on 
AFAIK is just 0. With that in mind, it’s probably best to use 1.22 for now if trying this out, but this does unfortunately mean it’s only able to decode assets added pre-WLM as it only supports the MSN version of the format. No harm in trying either though.