Be sure you have your Excel activex controls properly registered in your system, there are lots of activex registerers in the web. If this does not work, look at set cell value vi and check for invalid refnums. If even this fails, i'd try reinstalling app builder, but be sure you install it before any LV update (for example 5.0.1f1) unless you want problems like i had with Excel built examples. You can also send an example, and see if i can fix it, but i don't have excel 2000 (only 97) so i don't know if i can do anything else.
Hope this helps