The problem with the tools you have suggested is they aren't necessarily the best at detecting all things, at all times.. Avast is a fairly good VIRUS detector and also finds some but not all spyware. Spybot is a little outdated and doesn't seem to detect or remove some of the recent spyware items and it doesn't detect viruses at all. McAfee site advisor doesn't detect items that are already on the computer and as with most scanners, it depends on definitions which are "after the fact" and may not find new malware.. The others you mentioned are the same.. It depends on when and how they are used and whether the malware infection is something newer than the scanner is programmed to find.
As to how a tech knows if there is a virus... Sometimes using a different set of scanners tells them.. Sometimes they don't know for sure it's a virus. It's simply easier, cheaper for you, and faster to reformat and reinstall everything from scratch.. As just an example, I fix infected computers as a side job.. Many times, the customer wants all their important data saved from a computer that barely runs.. In one such instance, it took me two days of running scans and repairing the operating system to get it back to a clean condition while still saving the files.. A reformat and reinstall takes a few hours.. They PAID for the two days worth of time.
Hope this helps.
talking with my brother about a problem I have with the RAM in my pc,he told me:"Maybe you have a virus." No,I don't.Avast,Spybot,McAfee SiteAdvisor,HiJackThis and my config keep all that at bay."Mmmm...last time my RAM was bad the technician told me I had a virus although my NOD32 didn't find it.I had to format the HD"
Can this be true? Even with the best up to date antivirus,passing all the tests,HJT and everything? Can be? And if so,how technicians can be able to know we have a virus? Do they have a magical antivirus as nursery growers have magical ingredients to make plants grow and bloom bigger and better?