***
Welcome to the forums, zhouzhaoxu
You may not have had to uninstall/reinstall avast. The first thing to try when you have an avast functioning problem is a repair of avast. You do not say what OS you are using but the below is for XP. Other Windows OS will be similar.
To do a repair of avast :
Control panel > Add/remove programs > click once on avast to select it > click on Change/Remove > scroll down the left pane and click on repair > click OK > follow directions
You can try that now though I am not sure it will help ... but it will not hurt to try.
If you still have a problem after, please post in this thread again.
***