CRE loaded Malicious Code Removal & Security Fix

We have been repeatedly hacked last few weeks and keep restring the sites from clean back ups. The Problem is soon as its restored 24-48 hours later virus appears on the site. We need to clean any malicious codes from the websites and update Security Fixes or step this up from further prevention. Fixing any security holes that make it easy for hackers to hack into the online cart. Further Security prevention is also welcomed to lock down the site from unauthorised access of changing files or injecting Malicious codes. We are running 4 Websites, All sharing one Data-Base and using one Admin for backend access. The other 3 sites admin panels are accessible but never used. This project Budget is around 150 USD Approx. Payment via escrow.