For Excel 97, Help, Workbook let me see the following:
"Guidelines for naming workbooks
To make it easier to find your workbooks, you can use long, descriptive file names. The complete path to the file, including drive letter, server name, folder path, file name,and a three-character file name extension, can contain up to 218 characters. File names cannot include any of the following characters: forward slash ( / ), backslash ( \ ), greater-than sign ( > ), less-than sign ( < ), asterisk ( * ), question mark ( ? ), quotation mark ( " ), pipe symbol ( | ), colon ( : ), or semicolon ( ; ).
If you work with other programs that do not support long file names, you should limit file names to eight characters."