I believe that Bitdefender Antivirus Free Edition is the best antivirus solution for Windows, but there may be times when you struggle with specific applications which don’t like coexisting with antivirus programs.
I am specifically thinking about QuickBooks now, deployed on a network, and used in multi-user mode by various people at the same time. Your ongoing fine-tuning to whitelist the multiple processes and exclude the relevant folders from being scanned to keep QuickBooks happy can get very tedious. Even if you get it working once, it could suddenly stop working again after some time.
An alternative to this drama could be to use a vanilla antivirus program which won’t interfere with a demanding application such as QuickBooks. You may even have to appease Intuit’s Tech Support team while troubleshooting your QuickBooks issues on your “server” PC.
If your “server” PC runs Windows 7, then you could run Microsoft Security Essentials as your antivirus solution. Intuit’s Tech Support team encourages this protection although you may feel that it does not offer sufficient protection against evolving threat vectors. If your “server” PC runs Windows 10, then you could run Windows Defender Antivirus as your antivirus solution.
Running Trend Micro’s HouseCall Free Online Virus Scan is always useful, even when you have other antivirus protection in place, it provides a second opinion since malware is often responsible for a PC’s strange behaviour.