That has nothing to do with the notebook being very old. That's just a stupid bug in how InputForm communications between kernel and FE happen. We already had an open bug report about this, and I just pinged the bug again with a reference to your post. Here's a way you could work around the bug:
John Fultz email@example.com User Interface Group Wolfram Research, Inc.
On Mar 7, 2013, at 9:49 PM, firstname.lastname@example.org wrote:
> I opened a very old notebook (done in Mathematica 4 some 10 years ago) under Mathematica 9.0.1 running under Mac OS X 10.6.9. The InputForm output of Greek symbols is mangled. For example > > Print[\[Mu]//InputForm] > > produces a circumflexed I followed by the 1/4 fraction. Cutting and pasting that output into an Input cell reproduces that garbage. The problem seems to occur only with that filter, which unfortunately I use a lot for inlining code. > > Is there an option that can be set so that the Greek symbols are respected? > Thanks. >