[Geek] Rasn snafn rasn .ZIP rasn

Can anyone recommend a solution for corrupted .ZIP files? We've got one which is telling me that it's either "not a valid ZIP file or last volume of spanned ZIP set, or the structure is damaged". Obviously, free solutions preferred, but in extremis anything would be considered (I may draw the line at self-sacrifice, but I'm sure I can nominate some possibles ...)
Cheers!

EDIT: Downloaded the nifty, free, Zip-Repair from www.zip-repair.com, which managed to recover 107 out of 108 files. Rar!
EDIT 2: Except that it lied - although it appeared to recover those files, I'm still getting the same error message - and this shouldn't be a spanned set, because it's just one single file on a hard-drive. Hyalp! Hyalp!