I too had issues when upgrading from XP Pro to Vista Ultimate (a BSOD). After much searching, I found that many people recommended upgrading ALL drivers (including the BIOS), even if Vista Upgrade Advisor says they are OK. After upgrading all of these components, I had no issues.
For those of you who have received your Vista upgrade for an HP computer, take note. You may need to upgrade the soundcard driver. After installing the upgrade, I went to the HP site and upgraded the provided drivers. There was not update for the sound card. I discovered my Mic was not working. I went into device manager to upgrade the driver and chose the option to look on the Internet for a driver. Sure enough, there was a better driver. Now all is owkring well. The soundcard is no longer using a generic driver. It is using the driver provied by the manufacturer.