The fix mentioned above didn't solve my 503 service unavailable problem after installing Service Pack 1 (SP1) on Vista Ultimate x64. I had to uninstall the update for windows (KB947562) and uninstall SP1 using the control panel. I think if you install updates after SP1, it doesn't let you uninstall SP1 anymore. That's why I uninstalled KB947562. After uninstalling SP1 (which took about 20 minutes), I reinstalled IIS7. This may be an extra step, but I did reinstall it and it works fine again as long as you had a backup of the administration.config and applicationHost.config files located in C:\Windows\System32\inetsrv\config. I replaced these with my backups at the end and restarted w3svc and iisadmin.
Microsoft won't support this issue for free. I tried using their free SP1 chat and they told me to go to the paid support service. Microsoft wasted 3 - 5 hours of my time on this issue. I hope someone else figures it out soon