How do I open XLS files in Numbers? I've just bought Numbers and I'm trying to open some XLS files.  They're in Excel '97 format, not encrypted, and with no password set.  However, when I try to open them, I get this error message:

"The document is encrypted and can't be opened"


They open fine in openoffice.  How do I open them in Numbers?
Sometimes a xls file don't have password to open it, but it has it to avoid modifying certain fields, you can validate that. 
Also, if you have access to a PC with Windows you can try to save your xls file in a newer format.

Another alternative is to use a program like parallels or VMware to install/emulate window on your Mac.