When trying to delete a site collection, the following error pops up:
Command: stsadm -o deletesite -url http://servername/sites/sitename
Error: The system cannot find the path specified. (Exception from HRESULT: 0x80070003)
An error like this prevents you from upgrading SharePoint aswell, because you won't be able to access the site collection either. I found out the hard way.
To resolve the error, detach and attach the content database of the afflicted site collection.
stsadm -o deletecontentdb -databasename wss_content_name -url http://servername
stsadm -o addcontentdb -databasename wss_content_name -url http://servername