najkiie, it's not just you. My mother's laptop has AVG on it and every time I boot it up, it starts downloading definitions and running scans. It slows down the whole system just when I have booted it up because I want to use it *now*, not 20-30 minutes later when AVG is done. It's very annoying.
I don't use AVG myself for this reason among others. I don't know the exact settings, but there is one that is something like "run a quick scan after an update". These quick scans are not very quick, from what I have seen. As for the definition files, I think this always happens to me because Mom doesn't use her laptop everyday. So when I get on it, the time to check for updates has elapsed and the first thing it does is to start the update process. I suppose the best way around this would be to extend the frequency of update checks, or disable automatic updates and do them manually.