Third party help is difficult because we don't usually get much information. Here are the things to consider for a slow computer:
1. Too many programs starting at boot and running in the background. Using the msconfig utility startup tab, uncheck all startups "except" the anti-virus program, firewall, touchpad if laptop, network process if on network.
2. Amount of installed RAM. Many users have too little for good performance.
3. Poor maintenance. This includes disc cleanup, error check, defrag, uninstalling all programs no longer being used.
4. Checking for spyware/adware/viruses> 1 AV program, 2 or more s/a Each should be kept updated and scanned with regularly, updating right before scans.
Most likely some or all of the above are responsible for the slow computer, not Avast as Avast is not known to cause a system slowdown..