Do I need more than one antivirus?

I’m using Avast! and have no problems. I’m just curious, would one or more additional antivirus programs, run consecutively, give me more protection? Would they conflict with each other? Thanks in advance,

Jake the paranoid.

Anti-virus programs tend to interfere with each other. You can run more than one anti-malware program.

Do not use more than one antivirus program at a time. First off, as stated above, they will interfere with each other (often in a way that will cause both of them not to function at all). Second, it’s a huge waste of system resources.

I’d personally recommend either Avast (which I use) or AVG for antivirus. Both are free, and both are excellent.

I’d also recommend Malwarebytes for anti-malware, and CCleaner for keeping your PC declutteted. Also both free.

Just to provide more detail:

Antivirus programs (and other security software) use a facility known as a “hook” - this is where a replacement function is substituted for a built-in Windows function. Typically this will involve hooking the file execution function. When this is invoked, the new function will first trigger a virusscan of the file, and if it passes, then pass control to the original Windows functionality (which will execute the file). If the file does not pass the scan, the hook returns with a error value of some sort (the execution whitelist system we use at work returns a “This file is not a valid Win32 executable” error code).

Technically, hooks can be stacked - each new hook chains on any previous hook until the base function. However, if you have multiple virus scanners, each scanner will take time to scan the executable. This slows down function execution, sometimes to the point that things fail just because they take so long. Also, the different errors generated by the antivirus scans can confuse the hooks sitting above them:

How it should work
Hook 2: Avast - scans notepad.exe, ok
Hook 1: McAfee - scans notepad.exe, ok
Windows: runs notepad.exe, cool
Hook 1: cool
Hook 2: cool

What might happen
Hook 2: Avast - scans newvirus.exe, ok
Hook 1: McAfee - scans newvirus.exe, uh-oh
Hook 1: not cool, 0-length file
Hook 2: huh - that never was a 0-length file, let me check, um … <crash> :smack:

If the hooks never make assumptions about lower layers and always just past result codes back up the chain, it should work. But this is not always the case. This stacked hook issue is replicated with registry access, file copy, and a host of other places, and should be avoided for performance and stability reasons.