What starts with Windows is likely to be one of the main reasons, and Autoruns is likely to reveal much about it.
2 AVs on a system is an absolute no-no. Some users "get away with it" for a while, depending on the combo. If they both detect malware at the same time, (which is the time you most need that protection) a tussle for control and system freeze is fairly likely, requiring a reset or restart, after which the malware, not stopped by either 'coz neither could complete the process, runs.
I suggest the following:
Download the following tools.
Avast uninstall utility.
AVG Remover.
If you don't have the install file for the AV you want to use (
choose Avast! choose Avast! 
) download that, too. Disconnect, uninstall both, run the uninstallers in turn, reboot whenever prompted, install the AV, reconnect, update.