I have Vista running on 4 machines, no problems. I am really enjoying using it.
It is possible to do a clean installation with an Upgrade version of Vista. Launch the Vista installation from within XP. Select Install Now and then select clean installation option.
Vista then stuffs everything from your old XP installation (and I do mean everything; software, drivers, and XP OS itself), into a folder called "Windows.old". Then Vista installs to the remaining free space on you hard drive/partition. When it's complete and Vista boots up, you can go to the folder called "Windows.old" and delete it. Presto! clean installation of Vista with no trace of the old XP OS.