This error only normally occurs during VPOP3 startup. It occurs if some other software running on the same machine as VPOP3 is using one of the ports that VPOP3 is already using. This could be a proxy like WinGate, WinProxy or anti-virus software that acts as a proxy, an LDAP server (e.g. Active Directory) or another mail server program (e.g. Exchange Server). The latter two causes are particularly common on server versions of Windows.
To prevent this problem you should either remove the conflicting service from the other software or change the port which either that other software or VPOP3 uses for that mail service. You can change the ports that VPOP3 uses for its various services on the Services page in the VPOP3 settings.
There are two ways to find out which program is using the conflicting port number. Each provides different evidence so if the results of one method are unclear try the other.
Open a command prompt and at the prompt type in telnet <ip> <port> where <ip> is the IP address of the VPOP3 machine and <port> is the port number which is in conflict.
The response you get back should indicate the name of the program currently using the port.
The alternative is to use a free tool called TCPView (originally from SysInternals) which lists all the ports currently in use for both TCP and UDP along with which programs are using them.