During installation, if the installer says that the VPOP3 Status Monitor is open, and won't let you install VPOP3, that means that Windows is refusing to let the installer overwrite the VPOP3STATUS.EXE program. Usually that means that it is currently running.
If you don't think the status monitor is running, it may be that it is running in another 'session' - eg a Remote Desktop session.
Run Task Manager in Windows, and tell it to 'Show processes for all users'. Then, check that VPOP3STATUS.EXE is not listed at all. If it is, you can end the VPOP3STATUS.EXE process(es) from Task Manager, and continue with the installation.
The installer actually shows that warning if it can't delete the vpop3status.exe file from the VPOP3 directory. Usually the reason for not being able to delete it is that that program is running somewhere. If you're absolutely sure it's not running anywhere, then you can delete the file fromt he disk manually before running the installer.