Check to see if it is actively scanning each file you open. It can cause some slowdowns. This is especially true if you have more than one AV or Malware scanner in conjunction with some extra start up programs (like messenger programs, printer software, updaters in general).
If this is not the case then....
Check for recent Windows and software updates, roll them back, and reinstall them.
Sometimes things can get goof'd and the program is running in the background eating up resources... may be trying to update itself. Check out your processes by going to task manager (assuming Windows OS).
On the to do list...
Remove bloatware and trial AV software.
Like aicom has stated you may be falling victim to bloat/crapware. Uninstall any software that you know is bloatware, this includes the 30-90 day free trial of Norton/McAfee/etc. Also be vigilant when it comes time to update Adobe (Flash/Reader), Java, or any other necessary software, they like to add software that you do not need.