What OS, Spk, bit ?
Your PC set up; CPU speed, RAM, … ?
Which Avast! version ?
How did you uninstall last AV ?
What other security programs are you running now ?

To exclude files and programs in the File System Shield. See image below
You can also do the same for Behavior Shield to include files and programs as trusted processes.

However, this may leave your system at risk since Avast! will not analize those files or programs. So you may try it but if nothing changes remember to delete those exclusions.