The administrator does not mean your account does not have to follow permissions on files, registry entries or restrictions. To sort this out I usually have to do more research on the error message but here's what I find most of the time. MALWARE DAMAGE.
Correction includes removal of the malware but that does not repair the damage. To repair the damage requires your skills at Windows 2000, the registry (most of the time) and google.
However http://www.google.com/search?hl=en&q=%22due+to+you+may+not+have+the+authority+to+install%22&btnG=Google+Search does not find that error message and it's not one I've seen.
I use Windows 2000 Pro, Office XP, IE6 on a computer built by a friend. My hard drive: Socket 370 FlexATX Mainboard with sis 630e Chipset; processer is C3 Samuel2 1 GigaPro CPU support 100MHz FSB (from invoice). I used Ad-Aware SE and attempted to update to Ad-Aware 2007 when it first came out and was not successful; so have not had it until CNET notified of new version. When I tried to install it this am I keep getting the message 'failed, due to you may not have the authority to install'. I went to systems and made sure that I was the administrator and used same password for user since I am the only one that uses this computer. It still gives that message. What do I do next? Grannylee