I also had to do this, except from a backup so I couldn't use the export option anymore.
Someone else already mentioned you need to copy the 'Your Feeds' folder, but that only gives you the folder structure and all messages that you already received. The subscription isn't in there so you won't receive any messages. Here is what you need to
do, just did it myself and I am very happy it worked for me.
First exit Windows Live Mail (WLM). Then copy the following from your backup or old computer to the same location on the new computer. Replace USERNAME with your own user name:
- C:\Users\USERNAME\AppData\Local\Microsoft\Feeds
Copy the entire folder, it contains your subscriptions - C:\Users\USERNAME\AppData\Local\Microsoft\Windows Live Mail\Your Feeds
Copy the entire folder, it contains your WLM RSS folders and received messages - C:\Users\USERNAME\AppData\Local\Microsoft\Windows Live Mail\RssFeeds.xml
Copy this file, not sure what it is, looks like an index of all the folders
Now start WLM again and you'll receive all messages again. I also received a lot of old and duplicate messages but at least I had my feeds back.
Next time the better option would be to make an opml export before reinstalling everything, or maybe even better bring all feeds online into feedly. Will have to do that anyway when I get Windows 8.x because that doesn't support WLM anymore and de Outlook.com
app doesn't have a feeds option.
Now back to the 782 messages I just received... :/
Hope this helps
John-Pierre