This thesis involves the comparative study of three popular antivirus software, Windows Defender, AVG and Avast. By comparing the system resource usage and basic threat detection ability of these antivirus software. The aim is to test what impact each antivirus software has on resource consumption, including CPU usage RAM usage, GPU usage, boot time of the system and transfer speed of files. All three-antivirus software was tested for detection capability by using the EICAR test file. The results shows that Windows Defender and Avast both where able to find the test file and quarantine it, but AVG could not detect it at all. Windows Defender had the least impact on system performance, AVG performed well overall but did poorly on threat detection, Avast had the most impact on system performance and was good in threat detection. This study provides users with the information to make right choices by realizingthe trade-offs between security and performance.