Try
www.malwarebytes.org in addition to your current solution. The free version just scans when you want it to, but the paid version will scan in real time. It works well with other antivirus solutions.
As for how they work around security, it really isn't that complex.
Scanning an entire file would be very time consuming so virus scanner tend to scan only a portion of the file, looking for known signatures of bad or malicious files. All the enterprising virus writer needs to do is alter his code so it doesn't look like the last version that the latest anti-virus software catches.