Vista unstable after installing service pack

I reinstalled my copy of Vista a few weeks ago. Prior to this I had used Vista without SP1 for over a year without any problems. Everything was running very smooth. Yesterday I installed SP1 and the first thing I noticed was that it took quite a bit longer for Windows to boot up. Then I noticed that my wireless network kept getting dropped every 30-60 minutes, which required a reboot to fix. I should mention that all of my drivers are up to date. My web browser of choice, Opera, became nonresponsive after a short amount of time, and so did a few other programs like Winamp and Media Player Classic. Using the End Process button in the task manager did nothing. I couldn’t even end the damn processes! I’ve never had that happen before, either with Vista (pre-SP1), XP, 98…

I decided to uninstall SP1 and now everything is back to normal.

The reason I put off installing SP1 for so long was because I was under the impression that it was just a collection of all the previous updates realeased up to that point. Is that true?

I have most of the other updates installed, so is there anything critical in the service pack that I would need?

Is it possible that a few of my installed programs were incompatible with SP1 and were the source of instability?