I have an old PDF.zip file that is password secured. Naturally I can't remember it. Is there a way to change it or disable it?
Some Zip archives use the old PKZIP encryption, which uses a weak homemade stream cipher. The attack is described here (implementing it is a nice exercise for crypto students, by the way). Tools which apply that attack exist (use Google to find some). Windows XP inherent abilities at "Zip password protection" use this weak algorithm; see this answer for some details.
Other Zip-aware tools rely on AES encryption, which won't be as easily broken. If that is the case, then your only hope is to try some brute force on the password, in case it was chosen by a human being and thus possibly in range of a dictionary attack. This may fail if the password is strong enough.