|Solution:||Re-enable User Account Control or install in XP Compatibility Mode.|
Installation Isn't The Only Problem
Even if you successfully install Reader, you may not be out of the woods yet. Reader 8 on Vista may fail to start, give you a runtime error, or hide behind a "please wait while the document is being prepared" message for way too long before it displays the PDF.
Both these problems, which also affected previous versions of Reader on Windows XP, are likely to be related to problems with plug-ins. If Reader won't start at all, find the plug-in called Updater.api (it's in the Program Files\Adobe\Reader 8.0\Reader\plug_ins folder) and disable it -- rename, move, or delete it. You'll lose some reader functionality that you probably seldom use anyway. If Reader takes an agonizingly long time to index the PDF before it displays it, try disabling the Accessability.api file in the same directory.
And keep an eye on Adobe.com for the promised update to Reader 8 that should fix all these problems.