OK it may be something interfering with Avast ... Lets see if we can confirm that
In the search box type Msconfig and select the programme that appears at the top
1.In the System Configuration Utility dialog box, click Selective Startup on the General tab.

2.Click to clear the Load Startup Items check box.
NoteThe Use Original Boot.ini check box is unavailable.
3.Click the Services tab.
4.Click to select the
Hide All Microsoft Services check box.

5.Click Disable All, and then click OK.
6. Now replace the tick in the Avast antivirus box
7.When you are prompted, click Restart.
Now run a scan ... does that work properly