This error message is displayed "when Google detects that a computer on your network is sending automated traffic to Google." Maybe you use an application that monitors the ranking of a site in Google's search results, maybe a Firefox extension sends automated requests to Google every 5 seconds or a different computer that shares your IP is part of a botnet.
Remedy - Google suggests to use an anti-malware application and to contact your network administrator if you still can't use Google. This is the perfect opportunity to try other search engines (Yahoo, Bing), but I noticed that you can circumvent Google's security filters by using a different Google domain: instead of searching from www.google.com, use www.google.co.uk, www.google.com.au or another international domain.
Source
