just tried 7zip, winrar, winzip, default windows unzipping utility for xp sp2 and all works without errors, must be your download or something
first of all, which version of flashback do you mean?
check if your archive has this file size
floppy version- 3 045 414 bytes
cd version- 89 475 807 bytes
if you wish,
here i uploaded the floppy version as self extracting archive. if it's the cd version that you're looking for, i won't be able to finish uploading that much before i go to sleep today, so ask someone else or just download it elsewhere