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I would research as to what cipher was used in zip files back then and see if it is currently defeatable, like RC4 or AES with bad padding, and then work through the cryptopals.com problems and see if any of this applies to the zip file.
You have either brute force with more characters or if you have a file in there with a known content there is a known-plaintext attack on zip encryption. I've used it once and it worked nicely but it's rather tricky to find another file with the exact same content for this.
wow! this is a good one! The Zip contains not only the logs but all the files that were part of the mIRC application (executable, config, etc). Maybe I'll try to find some old config files and explore this option.
> SureZip attack decrypts Zip archives created with WinZip version 8.0 and earlier in less than an hour regardless of password used to protect it. At least 5 simultaneously encrypted files are required in order to process the archive. Archives created with WinZip are supported.
I used this when it first came out and even with a 60-char password, if there were more than 5 files it could extract them within an hour (sometimes less than 5 minutes).