It depends. How's that for an answer? In my experience, call center don't need to have full-on workstations unless the techs are doing some kind of development work. VMware or Citrix on dumb terminals would be better. And Vista? What for Vista?
Anyway, for a while there when I was working as an IT guy I had a Dell XP laptop that took a good 15 minutes to boot. I never figured out why. I checked everything, disabled all the management tools, shavlik and SMS, stopped the vpn drivers from loading on boot and basically turned everything but anti-virus off. I used to manually clean out all my cookies and temp files before a shut down (it was a work computer after all) and I stopped mapping any network drives. It didn't matter if the damn thing was logging into the AD domain or not. Slower than dirt to boot. I even went so far as to suspend it, instaed of shutting it down, which is often a good deal slower to re-start than just turning the sucker off, and even leaving it locked and turned on at my desk, but management frowned. I called our desktop support guys, who gave me the phone number for Dell support, who told me that, "high powered workstations often take longer to boot, especially if they are being used for administration". I shit you not.
So I never did get the slow boot fixed. I left that laptop sitting on my desk when I left for a new job, and the last time I was in my old cube visiting friends, that laptop was still sitting there. Alone and unloved. Deservedly so.