your user account must have the
disk admin server role to attach/detach databases.
The security system of the MS
SQL server is quite complex and you need some time to understand it.
http://www.mssqlcity.com/articles/adm/sql70roles.htm
The local windows administrator has by default all rights on the
SQL server.
If you're in the context of an normal windows user account you don't have the rights to create/delete/backup/attach/detach databases.