My guess is that you changed the entry here:
Tools|Options|General tab|"At startup, open all files in:"
(xl2003 menus)
If you remember what you typed into that box, you can use windows explorer to
rename that folder (just temporarily).
Then open excel
change that entry (most people leave it empty)
Then back to windows explorer and change the folder name back to what it should
be.
==========
If you don't know the name of the folder, you can make a change to the registry:
If you're comfortable changing the registry...
Windows start button|Run|Regedit
Traverse to:
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Excel\Options
under: AltStartup
And delete that key.
(11.0 means xl2003, 10.0 means xl2002, etc)
But be careful. Since you're working in the registry, you may want to back
it up before you do any changes. (You don't get a confirmation prompt when
you make changes here!)