You may have inadvertantly blocked the site. Having tried the above, I wonder if you are using Norton Internet Security or similar? I would think that the problem could easily be caused, by Firewall, Int security, etc.
Were it me, I would first, just confirm with your ISP that they too can get Pprune loading up and then unload any Internet Security suites and reinstall them from scratch. (Though do make sure that you are offline whilst doing so ) With bells and whistles like parental control and other little goodies, it is quite possible to block the site. I know that you have turned off whatever you have, but in my book, I still think it better to uninstall and reinstall.
Failing that, I will give you a tenner for your laptop.
Keep the faith!
Conan