VPOP3 needs to be able to write to the Windows Registry at HKEY_LOCAL_MACHINE\Software\PaulSmith.
Usually, VPOP3 will run in the 'Local System' user account which has full permission to the HKEY_LOCAL_MACHINE part of the registry. However, it is possible to lock down permissions further, or to run VPOP3 in a different user account. In that case, you may encounter this error message.
To do this, run RegEdit, find the HKEY_LOCAL_MACHINE\Software\PaulSmith registry key (if it does not exist, create it).
Right-click that registry key, and choose Permissions.
Then, give the account which VPOP3 runs in, 'Full Control' of the HKEY_LOCAL_MACHINE\Software\PaulSmith registry key and sub-keys. If VPOP3 is running as the 'Local System' then you need to give permission to the SYSTEM user.
Once you've done this, you will need to restart VPOP3.