I'd heard a rumor that installing Windows Vista Service Pack 2 resulted in a dramatic reclamation of disk space, on the order of tens of gigabytes. Fascinated by this, I decided to monitor available disk space on both my notebook and desktop computers as I installed SP2. Here is what I found.
On my notebook, free disk space was 27.0 GB before installing SP2 and 47.1 GB after. That's a very impressive gain, given the size of the hard drive (it's a 160 GB drive with 140 GB in the OS partition).
Over on the desktop, running Vista 64, free disk space was 386 GB prior to installation and 470 GB afterward. The OS volume on this computer is 581 GB in size.
Those are both some rather remarkable gains in free disk space. And I was impressed ... until I stopped to consider where they might have come from and the obvious occurred to me.
All previous system restore points were now invalid!
Think about it. What does a system restore point do? It preserves system files and various configuration settings so you can restore your computer to an earlier, working condition. A Windows service pack dramatically changes and updates system files, and to restore these files from what is essentially an earlier version of Windows would be disastrous.
So, while the buzz on other blogs and in assorted forums is correct, you DO regain disk space when you install Vista SP2, I believe there is a very simple explanation for this. I believe that installing a service pack erases system restore points, and that is where the disk space is reclaimed.
That being said, my experience thus far with Vista SP2 is generally quite good. I'm seeing some positive performance improvements on both computers, although these are only anecdotal seat of the pants impressions and I have no benchmarks to back them up. I am, however, fairly well inoculated against the placebo effect because of my decades in the IT business and a healthy mistrust of updates. I never expect them to go smoothly, nor do I expect them to always yield performance boosts.
If you're wondering whether to install Service Pack 2 on your copy of Vista or Vista 64, I'd say that you should. Yes, it does mean that your copy of Internet Explorer will be updated to version 8, but if you've got half a brain you're using Firefox or Chrome already so the version of IE that's NOT being used on your computer matters little.
Just don't be amazed and awed by the increased free disk space when you do. It's an illusion.