All,
AMyers is exactly right on this one. The installer is expecting a hash generated by the UninstallPwdKeyGenerator utility that comes in the Agent Tools download. If you enter a plain-text password it treats that as the hash and you won't be able uninstall the agent via the MSI. The DLP Admin Guide describes this process (and usage of that utility) in detail.
For an agent installed this way you will need to manually uninstall it. Please open a ticket with Support for assistance with that process.
Let me know if you have any other questions!
Tim