These two articles give you step by step checklists for reasons why a site can be flagged, where to go to get reports about what is wrong, and how to resolve many of the possible causes. That should be a good start; then you can ask here for any needed clarifications:
How to remove ‘This site may harm your computer’ from your website’s Google results: http://25yearsofprogramming.com/blog/20071223.htm
How to prevent your site from getting hacked, repair damaged site. Website security precautions: http://25yearsofprogramming.com/blog/20070705.htm

This report may be useful. It claims that the payingpost scripts that you are using are known to be bad. I notice that when they are fetched, they do not come from that site, but from blogadvertisingstore, which is listed in its Google Safe Browsing report as an intermediary:
http://wam.dasient.com/wam/diagnose?scan_id=141035