I have a run of about 70 feet of CAT5 cable, of which 10 feet needs to
run in a small, confined box along with electrical power lines and
speaker wire. I'm wondering how to insulate the CAT5 data cable from
interference - we were thinking of placing a piece of plywood between
the data and speaker cables and the power lines. They will still be in
very close proximity though. Is there a conduit I can use to insluate
the CAT5 cables? Is this something I have to worry about?
Ben.

The plywood offers no electrical isolation. It would tend to keep the
wires physically separated, which would help a little. Personally, I
wouldn't worry about it. If this is for home use, just run the CAT5
and try it. If you find that you have lots of errors on that network
link, you could use standard metal conduit or find some way to keep
the wires physically separated.

Is this for a home computer network or is this for home automation?

I'd be worried about the speaker wires near the 120v lines.  You
'need' to keep 40-48" between class 2 and class 1 (high power, 120v)
lines.  Metal conduit can help, but if it's actually important (i.e
for a client), find EMI or EMF shielding from many places (incl.
Google :) and wrap the length plus some (on both ends) of where they
cross or are near each other.  Parallel runs are the bad ones and
cause the most interference (audible and for networks). If the runs
must cross, cross at 90 degree angle.
hth

I'd suggest the use of STP CAT5 cabling instead of the more common UTP cabling.