ax sent us a link to the installshield.is6.general newsgroup, which is currently discussing the nimda virus. Apparently, this may not be an issue limited to the Myth III CD because it is likely an issue with Install Shield. Rick Lowe writes:

I just found it tonight.
We run NAV CE 7.51
W32.nimda.enc(dr) found on ikernel.ex_
Wonder if this is due to a bug in the latest virus def update?
A few months ago after an update, NAV found a virus on bat files.
Even ones on the NT Resource kit CD.
The next update which they put out quickly fixed this problem.
Hopefully this is what we have here.

ikernel.ex_ is the infected file, which is on the Myth III CD as well as in the Installer Shield files.