Not all antivirus and security programs are created equal. What works well for one person may not work for someone else.
It's possible that Avast doesn't play nice with your system. The only thing I can suggest is for you to choose another antivirus, uninstall Avast, install the new one, and see what happens.
If it works well with the new one, Avast was definitely causing the issue.
I tried McAfee once, it made my computer slower than a turtle.
Panda also made it run slow.
My daughter had one of the big name programs a few years ago, and it affected the performance of her system.
Uninstalling the programs cured the problem.
There are some excellent free anti-virus programs. A list of them can be found here: http://www.bleepingcomputer.com/forums/topic3616.html
If you want to stick to a paid program, the best thing you can do is see if there is a free trial of the program. If there is, install and evaluate it. It's better to find out if a program works for your system before shelling out money for it.