My site has been labeled as 'This site may harm your computer'. What do I do?
Clean up your site. If you don't know how to do this, contact your web host for help. These resources may help:
http://googlewebmastercentral.blogspot.com/2008/10/malware-we-dont-need-no-stinking.html
http://googlewebmastercentral.blogspot.com/2008/04/my-sites-been-hacked-now-what.html
http://googlewebmastercentral.blogspot.com/2008/08/hey-google-i-no-longer-have-badware.html
http://www.google.com/support/webmasters/bin/answer.py?answer=45432

I've cleaned all the malware off my site. How do I get rid of the label?
Request a malware review in Webmaster Tools:
https://sites.google.com/site/webmasterhelpforum/en/faq--sitemaps-malware--other-issues#malware-review

Protect yourself from this situation in the future:
http://googlewebmastercentral.blogspot.com/2007/09/quick-security-checklist-for-webmasters.html
http://googlewebmastercentral.blogspot.com/2009/02/best-practices-against-hacking.html