I also face a similar problem. I added the latest mcafee definitions, which I extracted from the latest SDAT, to the UBCD ISO. When I boot with UBCD, and try to run the mcafee command line scanner, it says 'unable to load scan86.exe'. Nowadays, the McAfee definition file is almost 50MB, and the program may not be able to load the definition in memory.
A workaround I tried was to start the system in Safe Mode with command prompt, then run the scanner, which loads correctly, as Windows is managing the memory in this case. It works, but then the whole concept of having a boot disk is eliminated, as we need a boot disk because we dont want to load windows.
I think the UBCD must be tweaked to be able to use all of the physical memory installed on the system, which will enable it to load the definitions.