Couple of things to check
Hi,
Can't say this kind of thing has never happened to me before. Here's some maneuvers that have worked in the past.
First, if you're running anything from Windows 95 to Windows ME (not WinNT, Win2K, and WinXP) you have to do a "hard" reboot (power off, power on) everytime you get that message or other stuff is liable to fail as well!
Second, try copying and "paste special" "as values" into a second sheet.
Third, if that doesnt work try saving the worksheet as an ACII file (warning, only saves the active worksheet)
Fourth, if that doesn't work, try copying the worksheet and pasting it into another application like notepad (if the file is small enough) or Word or Wordperfect.
I've also heard that you can open damaged Windows-based Excel files by opening them in the Mac version of Excel- http://www.accountingadvisors.com/exceltip.shtml#damaged
hope that helps, and good luck!