Sometimes, while browsing your website, you might encounter a warning message such as
Visiting this web site may harm your computer! ( on Internet Explorer )
Reported Attack Page! ( on Firefox )
Here is the way how to fix this:
Common reasons for which Google marks certain web site as harmful are as follows:
[b] Your site is hosting phishing page.
[a] If you have installed updated version of third party scripts, template, theme etc.
[b] The third party scripts which you are using in your web site are not secure.
[c] Your FTP password is compromised.
[e] The system from which you are managing your web site is infected by Malware, Trojan, Spyware, Virus etc.
[f] Insecure folder permissions set in your web site.
To remove “Reported Attack Site!” or "Visiting this website may harm your compuer !" tag from your web site at the earliest, use Google Webmaster Tools and analyze your web site.
Using this tools, you can easily find the infected pages of your web site.
Once after removing those infected pages, you can submit a request to Google for removing the tag.