Hey, I'm thankful you took the time to guess at all!
I figured out one reason why it didn't work for me...I had to use double quotes before s and after p instead of the single quotes.
Once I switched those out, your first example worked for me.
Your second example is actually closer to what I wanted, but I couldn't get it working. Let me offer some clarification (which I'm sure you'd have appreciated beforehand...sorry! hard for me to think in programmer terms)
After finding "ISS" in position x, that instance should be replaced with 3 empty spaces, and everything following should be retained and written to the new file. All other instances of ISS in that line are retained as well.
I should also mention I'm using sed utilities that have been ported to Windows. The two I'm playing with were found below:
http://users.cybercity.dk/~bse26236/.../SED.HTM#11.80
When I try your second script using this one, I get this error: sed: garbled command s/^\(.\{86\}\)ISS\(.*/)$/\1\2/p
http://sed.sourceforge.net/grabbag/ssed/
With this one (super sed) I get this: ssed: -e expression #1, char 25: unknown option to `s'
Maybe these ported version aren't as good as the real thing? If you have any ideas, I'd greatly appreciate it. And thanks again for your help.