I had a problem installing Macromedia Flash Play v10 in a Windows XP environment. The problem was that Macromedia Flash installed appeared to lack the permissions to install a certain file even though I was logged on an administrator. After a frustrating week I finally worked out what it was. :)

My problem turned out to be my McAfee Antivirus. Turning off ‘ ON Access Scan At Start-up’  allowed me to install the flash player. Remember to turn it back on post installation.

Another thing work checking are the permissions in the registry key HKEY_CLASSES_ROOT

Good luck!