Update on the Power Generator vs. Shield Generator controversy:
After several viewings of the battle on DVD, it has become clear that the Power Generator and Shield Generator are different, but related.
What we see Veers destroy is the Main Power Generator, which in addition to supplying power to the base is also providing power to the Shield Generator, which we do not see but probably looks like that dish on Endor in Episode VI.
Therefore, by destroying the Power Generator for the shields, Lord Vader can land the 501st and orbital bombardment can also take place.
Of course, the reason the Rebels were able to get the shield up was because Ozzel came out of light speed too close to the system.