Why do I need to reboot computer to connect to internet?

Someone help!
I have 2 computers (side by side) connected to a cisco
pix 501. I have cable internet for these 2 computers.
The one on the left never has problems connecting to the
internet. The computer on the right always has to be
rebooted, even though my icon on the taskbar says it is
connected at 100mbps, in order to view any internet
site. I get the "Cannont display page" error message.
However, I know it is connected because I can timbuktu to
a server that is located 30 miles out.

Please help. I am new to networking and would really
like to understand this problem.

Thank you!!