Internet access lost unreconstructable after certain amount of tim

I have the weird problem that my Vista Home Premium loses its access to the
internet after a certain amount of time. Sometimes after 3 hours, sometimes
after 5 hours or even after only 30 minutes I am not able anymore to surf
with any browser. I always get connection errors in every browser. Strange
thing is: According to my neworking center in vista the PC is connected to
the internet and I am able to ping IPs and websites with 'cmd' successfully.
ICQ runs successfully. Only browsing does not work. I tried many things like
deactivating the LAN adapter and reenabling and things like that. Nothing
helps. This problems appears with a LAN connection, with a WLAN connection
and on every router I connect to. The only way to get things working is
rebooting the PC. So I think it is not the fault of a certain driver. In
German forums there were people with exactly the same problem but no one knew
a solution. Can anybody help me?