Hi Will,
First off, I'll offer that it's a little unusual for files to become corrupted that frequently. You might want to verify that you aren't having any hard drive issues.
For example, do you see anything unusual after running "dmesg" from the command line? Sometimes when a disk is having problems you'll see errors towards the end of the dmesg output.
That said, the file /usr/share/webmin/web-lib-funcs.pl is part of the Webmin package.
You can download the Webmin source here:
http://webmin.com/download.htmlVerify that you're downloading the right version -- it sounds like you're running Debian or Ubuntu, meaning you can check what Webmin version you have by running:
dpkg -l webmin
After downloading Webmin, you can unpack that and copy the web-lib-funcs.pl over top of your corrupted one.
-Eric