You might try browsing in your registry. I have only Office 2003 installed; I have the following registry entries:

HKEY_CLASSES_ROOT/Excel.Application

with a sub-entry of

HKEY_CLASSES_ROOT/Excel.Application/CurVer

the Default value is "Excel.Application.11"

Under

HKEY_CLASSES_ROOT/Excel.Application.11

is found the link to Excel for automation. If you have something that points to a previous version (2000), then you might find it here.