My guess is that McAfee is hitting up against the 640k - 1MB memory range that all programs require a piece of to launch in all of the Windows 9x platforms.

Manually try setting your script to run from either a startup group, or the RUN key in the Registry and see if it works. If so, then I would guess it is the memory problem. i.e. something is either fragmented in this memory range or something is stealing too large of a footprint of this memory and won't share it.

Let us know how it goes.