never run more then one AV
your computer will be slow, get mysterious windows errors and false positive detections
so uninstall one, then run the removal tool and reboot to clear any leftover files that may conflict
http://singularlabs.com/uninstallers/security-software/
This is the generally accepted opinion. However, it is not exactly correct.
It is okay to run a two AV programs if the second one installed recognizes that you have another AV program running and the second AV turns off its real-time scanning. So one may ask what is the point of installing a second AV program if it turns off its real-time protection? Depends on the AV program. If it has many other useful features you may want to install it; e.g. auto AV definition updates and scheduled scans among numerous other features that are not in the avast! AIS 7. Also by installing the second program you can run on-demand scans without having to manually run the program and wait while it updates its AV database.
I have been running such a combination for months; avast! AIS 7 in real-time detection mode and brand X in compatibility mode. They work great together and I have no slow noticeable slowdown in my computers.
So it is true that should never run two AV programs together in real-time mode, however running two AV programs at one time works dependis on the AV programs and whether one of them turns off its real-time scanning when it is installed.
Others may have a different experience