The goal then is to encapsulate a PC, with software,
that
will survive 100 years of storage (probably a helium or argon atmosphere so
oxidation won't be a problem). Ideally the machine should be recoverable to
the point where you could boot an operating system on it and write a
program and run it on it.
I wouldn't expect a hard drive to be workable after 100 years of storage,
even in an inert gas atmosphere.
My first reaction was to suggest that you put the software onto a flash disk,
but that can't be guaranteed to last anywhere near 100 years either, since the
electrons slowly leak from the floating gates.
Only about 75% of my floppy disks from 1977 are still readable.
CDR media is predicted by some to last more than 100 years. But I don't think
a CD-ROM drive will last that long.
Punched mylar tape? That should last hundreds of years. Of course, it's
not very representative of how computers are actually used in the last
decade of the 20th century.