How to detach a sql server 2008 database that is not in database list?
I'm pretty sure the problem is that due to the uninstall/reinstall of SQL Server, the database engine no longer has permissions on the database files.

If you move the .mdf and .ldf files to the data folder that you specified on install, it should work for you. If you want to keep the files where they are, make sure the permissions are configured to allow appropriate access to the database engine service account group.