[masraum]

Some updates are security updates, so while not completely necessary, probably advisable.

Other's are just product updates for new or improved functionality. Java for instance, you could probably go a long time without updating java. But then Java also is one of the main languages that is used on the web, which means that it's updates could conceivable be security related.

As far as itunes goes, if you don't have any problems with iTunes, then you don't need to update it. Because of communications that iTunes does with other computers/devices in the area, its updates could also be security related.

I'd definitely recommend installing any and every update for Avast, and most of the Windows updates. Most big virus/worm/trojan outbreaks that occur, occur after Microsoft has issued the fix in some sort of update.

Anything besides those two is probably fairly optional, IMO.