1. Try this first , if you have not yet done so , Delete all cookies, temporary Internet files and browser history.
2. This is a common message many websites show when you are accessing them by a proxy. Do you use a proxy?
3. Do you have a Static IP or Dynamic IP ?
4. What’s your browser? Chrome? Looks like it is.
The Problem, At Least Right Now
If you heavily use Google Search in Chrome, Google doesn’t like you,
http://www.howtogeek.com/howto/28671/how-to-enable-google-instant-search-in-google-chrome/
5. Check for malware on your computer.Malicious software, sometimes bundled with other free downloads without your knowledge, can trigger Google to show this message, I like to use HijackThis a free utility that generates an in-depth report of registry and file settings from your computer
After using HijackThis, I suggest that you update and run the 2 or 3 if Mac user , free applications listed below .
* Lavasoft Ad-Aware
* Spybot Search and Destroy
* MacScan (for Mac users)