If it is the Poser-in-Program Files Folder scenario, then UAC will deny certain write back procedures in an effort to prevent malware and virus infections to primary software apps.  Ways to overcome UAC is either turning it off completely (not adviceable), giving yourself higher authorization on the app level, or reinstalling Poser into it's own folder on the core drive.

I might also add that the location of your content may have a bearing on how UAC responds to Poser usage.  One of the choices for installing P8 content is your My Documents folder, which steers clear from the Program Files folder and authorization issue.