Possible reasons:

1. You might have got a blend of office apps. i.e Some office apps are 2003 and some apps are 2007.

Fix:
Knock out all the office Installations and isntall a fresh Office 2007.

Reason:
Moss uses 'access' components to open the datasheet  view. Sharepoint 2003 used Excel components.

2. Do not have enough permissions on the list.

Fix:
add, delete, edit  and view permissions are enough for the standard view.
But, datasheet view requires 'BrowseUserInformation permission'. So, if you are using custom permissions.. go a head and check the box.