How do we know that particular anti virus software is good or not or how how to test the antivirus software.

These are tested programs by computer users worldwide -- Run scans and save reports

Online Scan
BitDefender Online Scanner - Free Online Virus Scan

Online Scan
ewido - anti-spyware and anti-malware solutions

AVG AVG Free Advisor - Installation files & documentation

SPYBOT
Spybot - Search & Destroy - Reviews and free Spybot - Search & Destroy downloads at Download.com

SpyBlaster
SpywareBlaster download and review - prevent installation of malware from SnapFiles

Run scans in Safe Mode with networking -- Scanning takes a long time - start it and let it complete the job

Copy and paste info in Note Pad for future reference -- Maybe you can help someone else someday