Hi,
For older versions of Geovision, like 6.xx, if you're getting the message "Decoder not found" you have to delete the codecs for that version and install it again.
For version 6.0x - The codecs are probably in this folder: C:\Windows\Downloaded Program Files - then delete files "RPBX" and "LiveX"
For higher versions - C:\Windows\v7000...v8000...v8200
Cheers