Avira and Avast anti-virus programs will work against each other instead of together if you run both at the same time as system monitors. Avira and Avast offer free versions of their security software for home users, but inclination to use both programs at the same time is counter-productive. It doesn't work like two layers of security: Avira and Avast will conflict with each other on how to handle identified infections while bringing down the computer's performance. The safest option is to pick only one.

Computer Performance Effects

Avira and Avast will scan every file against all known viruses and malware in each program's respective library. Avira and Avast have overlapping virus definitions and will miss blocking only a handful of potential infections each: the number of viruses that one program will block that the other will miss is minimal. When running two anti-virus programs at once, the computer will scan every file twice, while running a single anti-virus program shouldn't drop system performance enough for the user to notice. The computer will have to jump between Avira and Avast tasks if both are running at the same time, which further hurts performance.

Double Anti-Virus Arguments

If Avira and Avast both flag the same file as malware, they may disagree with what to do with the program. For example, Avira may want to delete an infected file, but Avast may want to quarantine that same file. Avast may quarantine the file first and prevent Avira from being able to delete the infected file, which can cause an error in Avira. Additionally, Avira may identify a virus and clean the infected file, which is an act that Avast might identify as an infection.

Tag-Team Antivirus Setup

You can certainly install Avira and Avast on the same computer for added protection, allowing them to work like a tag-team to keep the system clean. However, run just one of the anti-virus programs as a system monitor at any given time: the monitor function is the active scan of every file that goes through the computer. The second program can be used periodically to run system file scans to clean up anything that the primary program missed. For example, you can run Avira at all times while disabling Avast's monitor program and running a system scan with Avast monthly.

Assistance from Anti-Malware

Anti-virus programs are designed to go after the most immediate threats. There's a lot of malware that doesn't destroy files or compromise your computer but can bog down performance and alter program behavior. Anti-malware programs like SUPERAntiSpyware, Malwarebytes and Spybot are designed to clean up less serious malware infections. Anti-malware programs do not run in system monitor mode: they can be used only through on-demand scanning. You can install several anti-malware programs at once and periodically run system clean-up scans without conflicts.