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.
Edit:
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.
Best Answer
No antivirus package is perfect. I had seen viruses which I submit to http://virusscan.jotti.org/en and only 2 or 3 of the packages detect them. I have also had a virus which was reported clean by them all.
So, if I need to clean/scan a machine for virus, this is some of the things I do.
Prelimary Check
Check and possibly delete the files in the temp folder and also temporary internet files. If there are ten of thousands of files or more, deleting these can significantly reduce the time it takes to perform a full scan. It is however possible for this to delete a virus stored in these locations before it can be identified.
Stage 1
Boot off a clean CD/DVD for example a Bart CD or a special AntiVirus CD
Stage 2
Boot in the operating system normally
Stage 3 (time permitting)
Stage 4