If you clicked on a link from Google’s search results and you were taken to a page that tried to download a virus or other nasty then you can ask Google to remove the link. Instead they may decide to put a warning in the search results, such as “this site may harm your computer”.
Google uses various methods to detect and prevent malware (malicious software) from cropping up in the search results, but as they don’t scan each web page all of the time, you may occasionally see the odd nasty.
It is worth noting that if you are seeing strange redirects after doing Google searches, it could be a sign of malware already on your computer. This page may assist you with Google redirects.
To send Google a malware report you need to use this form – Google Malware Form.
Even if you have anti-virus software installed, it may be a good idea to do some deep/full scans if you recently landed on a suspect website. No security program can offer 100% protection so consider trying a few additional scanners.