It depends how long you have been a Ghost customer, but the answer may be that the system in question had a client from Ghost Enterprise 7.5 installed on it. For Ghost Enterprise 7.5 although normal installation was done by a traditional install program which added Add/Remove Programs control panel entries, the early code in the 7.x console which performed remote Client installations over the network was ... different ... in this case there was no uninstall entry configured on the machine itself, no start menu shortcuts, no add/remove programs.
Several years ago when support on the GSS server for direct upgrades from some older versions was dropped, the developer making the install changes also removed
all the code to upgrade the clients from this particular, somewhat unusual case from the client installer (why, I have no idea, but it happened), although the code to detect this old version was left in place. A similar situation related to the consumer versions of Ghost such as Ghost 2003, which the Enterprise management versions don't try and cross-grade from - side-by-side installs of the consumer and Enterprise versions weren't allowed, although I doubt that's what you are seeing.
Regardless, the installer which is failing is likely detecting the registry data for an older client still present. If it's the Ghost Enterprise 7.5 situation, the registry data is somewhere under
HKEY_LOCAL_MACHINE\Software\Symantec\
I don't recall exactly what all the different installers trigger on in terms of registry strings, but you can look for any Ghost-related registry keys under that one, and after saving a backup copy of the registry data under there experiment with removing them to bypass whatever is blocking the installer.