avast (and other applications) does not work good in multi-admin systems. In fact, Windows allow but it's not the suggested configuration, instead, it's better an admin-only account.
Back to the problem, well, try to solve this issue, turning off Simple File Sharing, and then take ownership of the folder:
1. Turn off Simple File Sharing:
a. Click Start, and then click My Computer.
b. On the Tools menu, click Folder Options, and then click the View tab.
c. Under Advanced Settings, click to clear the Use simple file sharing (Recommended) check box, and then click OK.
2. Right-click the folder that you want to take ownership of, and then click Properties.
3. Click the Security tab, and then click OK on the Security message, if one appears.
4. Click Advanced, and then click the Owner tab.
5. In the Name list, click your user name, Administrator if you are logged in as Administrator, or click the Administrators group.
If you want to take ownership of the contents of that folder, click to select the Replace owner on subcontainers and objects check box.
6. Click OK.
You may receive the following error message, where Folder is the name of the folder that you want to take ownership of:
You do not have permission to read the contents of directory Folder. Do you want to replace the directory permissions with permissions granting you Full Control? All permissions will be replaced if you press Yes.
7. Click Yes.
8. Click OK, and then reapply the permissions and security settings that you want for the folder and the folder contents.
Oh, maybe you could temporarily disable the avast self defense module:
1. Right click the 'a' blue icon on system tray.
2. Programs settings > Troubleshooting (tab)
3. Disable self defense module
4. Try to add the license key in the other users.
