>Specifically, when I try to execute a DOS Run command, I am getting an error message indicating:
>
>'An extended memory manager already installed. XMS driver not installed.'
>
>This has only started happenning since upgrading from 6.0 to 7.0. Any ideas?

I'd guess that it's an OS error - it's probably spawning a new CLI with different memory parameters than the CLI that launched the app. I'd try removing explicit references to HIMEM.SYS from CONFIG files, and make sure that the PIFs of various apps are consistent in their use of XMS and DPMI.