I would try disabling UAC, reboot, login as administrator and see if it installs. The local administrator vs. a domain account (if applicable). I realize the MS correct answer is to have UAC enabled. After your done testing, re-enable UAC. :-)
Steve Schofield
Windows Server MVP - IIS
http://weblogs.asp.net/steveschofield
http://www.IISLogs.com
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Install, Configure, Forget