Filter a text file into a new text file
Use the findstr command:
findstr my-string filename > new-filefindstr /? will give you a usage message telling you how to use findstr (including the /V switch if you wanted to filter lines out rather than filtering them in.)
Or if you're familiar with grep and wishing you had it on Windows, install Cygwin. :-)
Powershell has some neat string comparison features. You can pipe the output to a new file.
^^ There are some better links as to how to use Powershell - but I can't find them this minute. If you go the Powershell route there are lots of examples on the internet.
There's a logfile filtering utility Logfilter (www.casstor.com), that can easily do this. You can define multiple filter strings as well as use regular expressions in your filters. I think it might be what you're looking for.