Thanks for getting back Jeff, information is a touch sketchey because this colleague is located 250 miles away. Having said that however, She's very experienced and I've talked her through various debug scenarios over the telephone. Basically AutoCAD reports the files being "successfully loaded" but then returns "unknown command" at every use.
The software in question is a collection of 12 "vlx" files containing approx 160 commands and even more functions. Some commands reference external files (all available) others however are very basic and even these don't work. Up until now they've all been working fine on this same pc, it doesn't appear to be "dwg" issue and a even re-install of Acad 2005 didn't fix the problem either.
I've scanned through a couple of forums including the Autodesk forums and although a few similar reports were posted there didn't appear to be any conclusive fixes. The vl-load-com was the first thing I checked (thanks Daniel), now I'm left wondering whether it's possible for AutoCAD to start-up without loading it's lisp interpreter.
I've had difficulty running and even installing AutoCAD on Compacs before now so I'm beginning to suspect that it may be a hardware conflict, but I needed to check with the collective experiences here before passing on bad news.
I will check that she has the latest Service Pack installed tomorrow but to be honest I'm clutching at straws so any suggestions would be welcome.