I noticve you've not had a reply in a couple of days so I'll pass it on to some tech team members and see if they can provide an answer.
On the face of it, it should be a simple case of copying the old database over to the new machine, just as you would for a backup; however there may be file structure differences for W7 and possibly PMB compatibility issues too.
Can you confirm the version of PMB you've been using on the old PC?
The export function of PMB does not export the database files (e.g. the modd files, in which the additional information like Rating and Label are stored). You can use the export function to export the content of registered folders to an external HD, but the additional information is not transferred.