First, I can get my PC to exhibit this type of behavior, but, it does take "planning and perseverance". Like this; Photoshop Elements running, a couple a 10 GB folders open, Firefox running, with about 70 (yes seventy) tabs open, as soon as I try to load about a dozen more tabs with 1MB or larger Jpeg photos on them while serially downloading other 1+MB photos, Mi "durn" compooter's HDD starts running and it doesn't seem like it's gonna stop, ever. XP with 1.5 GB of DDR RAM. Well, I know this is my own fault, so the reboot process doesn't hurt my feelings the way it used to.
So, statistically your machine should be less likely to act like this.
I have heard that video driver problems can provoke this behavior.
Have you gone through the process of using task manager to kill processes one at a time to try to determine if a specific culprit might be to blame?
So, video driver, program looping, program conflict, or pilot error, at least some things to consider.
When you say "Zone Alarm" do you refer to the firewall only or the security suite? BTW, it has been noted, (although I don't know if it's still the case) that the free Zone Alarm firewall, didn't play as nicely with Vista as one might hope.