Google search thinks I'm a robot?

This is the second time I've had this happen to me. Whenever I type something into google search I'll be redirected to a page that says:

"To continue please type the caracters below, (and it has a CAPTCHA for me to solve)

About this page

Our systems have detected unusual traffic from your computer network. This page checks to see if it's really you sending the requests, and not a robot. "

Then it has my ip address.

When I click the "why did this happen" link I get th following message:

"This page appears when Google automatically detects requests coming from your computer network which appear to be in violation of the Terms of Service. The block will expire shortly after those requests have stopped. In the meantime, solving the above CAPTCHA will let you to continue to use our services.

This traffic may have been sent by malicious software, a browser plug in or a script that sends automated requests. If you share your network connection ask your administrator for help -a different computer using the same IP address may be responsible."

Help.....what does this mean, I've only had this happen twice...