What would we do? If you think about it, the outcome's not as simple as the movies make it seem. Either they are much more advanced technologically and possibly intellectually or they had some kind of unfair advantage that allowed them to make their way into space faster than us.

Hollywood doesn't get paid to think all that hard on how an alien invasion would actually happen. Instead, they get paid to have high action and cool looking martians. The way the humans defeat the aliens is always highly unlikely (or highly impossible). Independence Day, for example, assumed that these advanced aliens used IBM compatible computers using Windows 95 and had an irresponsible IT department who installed no antivirus software. Otherwise, how would Jeff Goldblum have put a virus into their system?

In reality, it would come down to one of the two cases stated above: more advanced technology or unfair advantage that allowed them to enter space prematurely. In the first case, they would probably not only have more advanced space technology, they would also have more extensive weapons technology. Furthermore, they would probably know the risks of such an invasion very well and would have thus overcompensated on resources and firepower to make sure they didn't waste a such a long trip. In other words, in this case, we have no hope.

As for the second scenario where they had an unfair advantage that allowed them to enter space with either lesser technology (biological maybe) or lesser intelligence, we may have a chance because though they may have won the space race, they may not have won the weapons and science races. As for the weapons race, maybe they hadn't developed nuclear technology or even known that such power might even exist. And for the science race, they may not have intended to use science at all. We, on the other hand, could study them and maybe create a biological weapon that would give us the advantage. We may even be able to imitate their space technology and become superior in every way.

When it comes down to it, either they're smarter or they aren't. When they're smarter, they're going to win, when they aren't, we have a chance. Furthermore, we really have no concept of what they may really be. Personally, I liked Arthur C. Clarke's idea in Childhood's End where it wasn't an invasion at all.