If you go into start menu, accessories, command prompt and type
Why not Start -> Run -> cmd ....
Does this only happen with PPRuNe, or other sites too ? Does it happen consistently or intermitently ? (try running mr Optimistics pings with a -t flag to make them run for more that 5 or whatever the default is).
Have you tried rebooting your ADSL router ?
Is this WiFi or wired ? If WiFi, try plugging directly into the router and seeing what happens.
I was having PPRuNing issues last Saturday (intermittent dropouts), but a quick phone call got an engineer promptly despatched the same day to the local exchange to do a Tie Pair shift and problems seem to have disappeared.... line seems to have re-synced at a slightly better speed too.
Hopefully your issue will not require such a drastic fix though.