1) Insert the Window Installation CD - DISK 1. May refer this KB link for more details
2) Login into your VPS using remote desktop (RDP)
3) Go to Start > Control Panel > Add or Remove Programs > Add/Remove Windows Components
4) Go to Application server > Internet Information Services (IIS) > Enabled File Transfer Protocol (FTP)
5) Click OK until finished.
6) Click Next to proceed with the installation.
7) That it you have just installed FTP service in IIS6 manually.