I recently spent many hours troubleshooting a laptop that could not connect to the internet. The laptop exhibited no other unnatural behavior, and so my first thoughts were to try connecting to other networks, try a new NIC, etc... The question I posted can be found here with more detail. One of the first things I did was to check for viruses with MalwareBytes, eSet, and Panda Cloud Antivirus... All 3 scans were run separately and independently of one another, and no virus was found. I then proceeded to spend hour after hour troubleshooting, and in the end I just took the computer to a repair shop where it was discovered to have a virus.
My question is not subjective, I'm not asking what is the best anti-virus software to use. I'm asking how can I actually be certain I have no viruses when popular and generally effective anti-virus scans detect absolutely nothing?
In the past my routine would be to run through the list of running processes and start-up programs, and use online resources to try and find anything malicious. This routine seemed relatively silly to me in the face of all of these anti-virus programs, and I thought it would be more effective to run scans than to manually look on my own.
Obviously IT firms have some effective method of identifying viruses, and I doubt these companies are just running some virus scanner. Clearly experience would have led me to identify my own problem as a virus, but I feel like there are all kinds of ways an undetected virus can manifest itself, so I don't want to rely solely on experience.
I should clarify this a little bit. I'm not necessarily looking for some "ultimate" checklist of things to do to identify viruses, but clearly there are ways to identify them when our normal anti-virus scans fail, and I'm wondering what some of these approaches might be.