I've noticed that the MacLinkPlus translators that came with AppleWorks weren't fully converting my Excel docs. I also had problems with AppleWorks attempting to open my Word docs. I downloaded icWord and icExcel and now I can either just read the files, or save them as an AppleWorks file without problems.
A 30 day trial can be downloaded from Apple's OSX download site or from the websites linked above, and each program costs $19.95 if you decide to keep it after the trial period.
[Editor's note: I have no experience with either program, but know that there are some people out there who work to keep their machines "Microsoft free" and/or cannot afford the cost of Office v.X. If you occasionally need to read Microsoft files, icWord and icExcel might do the trick. There are limitations to what they can do, of course, so take advantage of the trial before making your buying decision.]
Mac OS X Hints