Files not opening

Version: 2008
Operating System: Mac OS X 10.5 (Leopard)
Processor: Intel

I'm running Office 2008 on my MacBookPro running OS 10.5.3. Some, but not
all .xls files created in Office X fail to open. These are simple
spreadsheets no different than many that do open. A dialog box flashes much
to quickly to read, the do***ent does not appear and does not show in the
window menu. No slashes in the titles or sheet names.