During installation, the installer program does some basic checks for conflicting software. This is software which is providing the same services as the VPOP3 software - i.e. SMTP, POP3 and/or LDAP services. For POP3/SMTP it will show the text which the conflicting software announces itself as.
Depending on the conflict and whether you know the cause of it, you can tell the installer not to do anything, or to make some changes to the Windows configuration, or the VPOP3 configuration.
Sometimes antivirus software (eg AVG) installs a POP3 'proxy' service on port 110. This will appear to conflict with VPOP3. However, sometimes the antivirus software will move/remove the POP3 proxy if it detects a real POP3 server starting. So, if this is the type of conflict you see, it can be worth leaving things unchanged, and see if the antivirus software resolves the issue automatically.
If you change the POP3 service port (eg to 5110) then you must remember to change the email client port settings accordingly.
Once VPOP3 has started, you can change the POP3 service port on the Services → POP3 Service settings 'Bindings' area.
If you are installing VPOP3 on the same computer as IIS, often Windows will have installed the Microsoft SMTP service component of IIS. This will conflict with VPOP3's SMTP service. Usually the best thing to do in this situation is to disable the Microsoft SMTP service as it is usually not needed. The installation program can do this for you.
If you decide to change the SMTP service port to 5025, then you must remember to change the email client port settings accordingly.
Once VPOP3 has started, you can change the SMTP service port on the Services → SMTP Service settings 'Bindings' area.
If you are installing VPOP3 on an Active Directory server, that will provide an LDAP service as part of Active Directory. You must not change the settings on that, or you may cause Active Directory to stop working. Instead, you should tell the installer to move VPOP3's LDAP port to port 5389 and change the email client port settings accordingly.
(LDAP is the 'Directory Service' for the VPOP3 global address book)
Once VPOP3 has started, you can change the LDAP service port on the Services → LDAP Service settings 'Bindings' area.