I had the same problem and resolved this without reinstalling.
If you don't have a recovery pin setup, you can get it to send to the number of your "First Friend".
If you don't have a "First Friend" setup, you can set this number in your Avast account on the website.
Disable anti-theft from being a device administrator and recover your PIN
1. On you phone <system settings>,<security>,<device administrators>
2. uncheck the name of the app that anti-theft installed as (is an android robot icon), if you have the right one, it should say something like "This administrator is active and allows the app to: Erase all data, Change the screen-unlock password, lock the screen, etc.
3. Select the <Deactivate> button
4. It will ask for your Avast pin (which you don't know), BUT it also lets you choose to send a recovery pin, it will send you your "First Friend" phone number and will be active for 30 mins. If you don't have a First Friend number set, follow the second set of steps below; "Set up a First Friend number via the web."
5. Start Avast, enter the recovery pin (which you got from step 4 above)
6. Click <Anti-theft><avast! PIN>
7. Change the Avast PIN
8. Set the phone number for PIN recovery
9. Follow steps 1. & 2. above to re-enable anti-theft as a device administrator
If you don't have a first friend setup...
Set up a "First Friend" number via the web.
1. Login to
https://my.avast.com/en-gb2. Select your device
3. Choose <Anti-Theft> and the <Settings> tab
4. Down the bottom is a <Change Settings> button, select it.
5. On the <Basic> tab, set the phone number of your "First Friend" and click OK
6. Follow the first set of steps above to "Disable anti-theft from being a device administrator and recover your PIN"
Regards
-KENT