Slow is very subjective, it depends on your system, ram, cpu, avast settings, programs running in the background. With little or no information it is virtually impossible to suggest a course of action.

I would however review what programs there are running in the background and stop those not required. I would also suggest running HiJackThis as it will show exactly what is running, use an on-line scan of your Hijackthis log file try here http://hijackthis.de/index.php to identify what is adware/spyware, not essential, etc.