Just some basic steps..
Ping your local IP (192.168.1.105). Ping your Gateway (192.168.1.1). Ping each DNS server (68.87.73.242, etc).
Things ok? Let's try a look-up. Open a DOS prompt (Start -> Run -> cmd). Look-up google:
C:\> nslookup www.google.com
You should see a section similar to "Non-authoritive answer:". If you do not, and instead receive an error message, then you are having problems with DNS resolution.
The last step I can think of is to try telnet (Start -> Run -> telnet):
Microsoft Telnet> open www.google.com 80
GET / HTTP1.1
(sometimes requires a blankline)
Did you connect? Get back any HTML? If so, your browsers could have some problems (Proxies, for instance).