One of those problems are that you cannot uninstall Exchange server while it contains Public Folder Replicas and you can't move them to another server because there is none.
Browsing the web I was finally able to solve this problem, for full artcile just follow the link and read the comments.
1. You'll have to keep at least one mailbox with administrator rights.
2. Open Power-Shell and enter the following commands:
4. Remove Exchange 2007 server from "Programs and Features" in control panel.