When you see the dreaded Google attack site warning, you should immediately email your hosting provider. Be sure to include your domain name because they will need to locate the virus that caused the issue and take steps to secure account.
Once the account has been cleaned and secured, please request a delisting through your Google Webmaster account. http://www.google.com/support/webmasters/bin/answer.py?answer=45432
Once you have reviewed your site and are sure it is clean, you can can submit a request for review. Note, you will need to verify site ownership before you can request a site review.
1. Sign in to Webmaster Tools with your Google account.
2. On the Dashboard, select the site you want.
3. On the Overview page, click Request a review and follow the instructions.