When VPOP3 is installed, the installer creates a Windows user called vpop3postgres. The PostgreSQL database service (VPOP3DB) is run as this user.
The user is created as a local user on all installations of Windows except where VPOP3 is installed on an Active Directory controller. In that case, it is created as a domain user (Active Directory controllers do not have local users, so it must be created as a domain user).
You should not remove this Windows user unless you really know what you are doing, and doing so is unsupported.
The vpop3postgres user is removed from the Users/Domain Users group so should have no permissions other than those specifically created by the VPOP3 installed (i.e. access to the VPOP3\pgsql folder).
The default password for the vpop3postgres user is "Nc6ACboDt2jVL6". It is possible to change this as long as the login details for the VPOP3DB service are also changed. Note that changing it may cause issues during upgrades (because the installer has to perform certain tasks by running programs as that user (which requires the password)), or upgrades may reset it back to the default password (to prevent any problems).