Connected to Internet, Can't Browse

I've been experience a strange problem on my Vista laptop for months.

I'll be connected to the internet, and eventually lose my ability to
open web pages in my browsers, but I'll still be able to ping servers,
resolve dns names, etc.

I use a virtual machine for work, and often within the virtual
machine, I will still be able to browse pages, but not within the
host.

Rebooting solves the problem. This happens periodically and is
obviously very disruptive. Help!