Your Wanadoo a/c will exist whatever you do with your computer...... So after reinstalling the BB modem you should be able to quote a/c details and password at the appropriate places when you re-establish connection with Wanadoo.
Sorry, ignore my last post, I've thought about this again.
Yes, you probably will lose your BB because it represents system/file changes made AFTER the time you will be restoring to.
However, after restoring, you will have to re-install your BB/modem software but you will not have to wait for Wanadoo to do anything. Your 'phone line is now BB enabled and System Restore has no effect on that.
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