It sounds like you have a file in either your XLStart folder and it's not an
excel workbook.

Or you're pointing to folder that contains non-workbooks in your
Tools|options|General Tab:
Alternate Startup File location

If it's in your XLStart folder, move it somewhere else.  (You can use windows
start button|find to search for your XLStart folder.

If it's the alternate startup location stuff, just erase that entry (most people
don't use it.)

If that's not it, Chip Pearson has some notes to help find the problem at:

http://www.cpearson.com/excel/StartupErrors.htm