I can't install flash player.I've tried the trouble shoot suggestions,nothing works.

It downloads,starts installing,then says"don't have sufficient privileges".I tried the suggestion and make sure my Internet options are correct.I'm suppose to be the administrator,I'm listing as one.I been using a old version for a year or more,and it works.I recently tried to use flash payer as usual,but it wasn't working at all.I tried updating,like many times before,Tried to install latest version,nothing.I keep trying the trouble shoot help suggestions,nothing.Short of reinstalling windows,what can i try? I got windows xp sp3,and use fire fox 22.I know I need to upgrade windows,but I have a old computer,slow,and don't have the money to up grade,please help?