Avast! hard disk access on startup, and strange scanning size

I used to use the trial period for Norton Internet Security when I first got my laptop. I changed to Avast when this ran out and I've been fairly happy - it only recently alerted me to some sort of "rootkit" that appeared to be related to Adobe Flash player 9 (Flash9util.exe or something). Apart from that I've had nothing. I'm guessing it uses less resources than Norton too, although people seem to think Avira is better (how much difference is there in resources?)

Anyway, my startup is virtually instantaneous. I mean, I type my password and once the desktop appears all of my system tray icons are loaded in a flash. But for a while longer [I'm guessing 1 minute, maybe more?] the hard disk seems to be going quite frantic. Since all of my startup processes have loaded, what could this be? Is this related to the new Avast update, perhaps the rootkit scanning on startup? I have Intel turbomemory too - I was wondering if it was filling it's Readyboost / Readydrive, but I don't think it was always like this before - making me think it was some recent change...?

Another question - Avast used to take about 2 hours to do thorough scan of my laptop, approx 70GB used. Now it's taking 3 hours and scanning 120GB, even though still only 70GB is used.... What's it doing!?! Is this normal or has something messed with my system?

Thanks guys - any help would be appreciated, no matter how brief