Vista Scheduler contents questions

I have had scheduled events start when I am in the middle of doing something
which has then bogged my computer down. Also it seems every couple of hours
something runs that hammers the heck out of my hard drives.

I use my computer primarily for games browsing the web, and was wondering
how many of the scheduled tasks do I really need to be running and how often
do they really need to be run.

Such as defrag is set to run once a week, I have always run it every month
or two in the past.