Vista - Internet Connection Problem

I have problems with my internet connection. After half an hour or more, i am unable to browse the web. The web browser shows that "website found, waiting for reply." The web pages do not load though, no matter what web browser I try. However, all my other programs can still connect to the web. For example, I was able to run Windows update and download an optional update. (I am right now on my virtual machine posting to this forum.)

After a restart, everything was fine, but the problem would always occur again. It does not happen to my windows 7 (I dual boot), but only to Vista.

Does anyone know why this is happening?

Computer:
Vista Home Premium 32 bit
AMD X64 Processor
Dell Dimension C521
2 Gig RAM, etc

I have a similar problem on my laptop. When I turn it on it works fine and after 15 - 20 min it looses connection and unable to reconnect unless I restart.

I have tried just about everything suggested by HP to resolve the problem however nothing has worked.

Computer does not list ANY wireless networks once it looses connection and the system starts acting up (slow response to open network window or Office document not responding).

suggestions?

Thanks