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I have tried several times to install MS Office 2000 from my 2 CD set and also by copying all files to my hard drive, using the option in Playonlinux, but I keep getting an error about CD rom not found. This is despite the number of the CD ( 990714_2005 ) being detected by Playonlinux . Any tips please on how to get this working. I am using Linux Mint 18.3 64bit

There is a chance the CD ROM has a DRM setup for the installation.  Probably not, but you never know.

If you copy both of the disks for a folder on the desktop, install it from there, but also make sure the drive is recognized by Wine.  You can open Wine Configuration and click the Drives Tab.  From there you can tell the D: drive to look at CD ROM drive or the folder you created.