Hi there! I understand that when you try to turn on the VPN in Avast Secure Browser, it causes the browser to shut down. Let's get to the bottom of this together.

To start, could you tell me if you have any other Avast products, like Avast SecureLine VPN, installed on your PC? Also, have you noticed any error messages before the browser closes? Knowing this will help tailor the solution better.

Meanwhile, here are some quick steps we can try to troubleshoot the issue:

1. Restart Your PC: Sometimes a simple reboot clears temporary glitches.

2. Check for Browser Updates:
- Open Avast Secure Browser.
- Click on the three-dot menu in the upper-right corner.
- Go to Help > About Avast Secure Browser.
- The browser will check for updates and install any available ones.

3. Clear Browser Cache and Cookies:
- In the same menu, go to Settings > Privacy and security > Clear browsing data.
- Choose to clear cached images and files and cookies, then confirm.

4. Disable Conflicting Extensions: Extensions can sometimes interfere with VPN activation. Try disabling all extensions and then turn on the VPN.

5. Check System Proxy Settings: Avast VPN may not work correctly if a system proxy is enabled. You can check this by:
- Open Windows Settings > Network & Internet > Proxy.
- Ensure no proxy is set or the "Use a proxy server" option is off.

6. Try Turning Off VPN via Settings First: Instead of toggling the VPN directly via the browser toolbar, try enabling it via the browser settings:
- Go to Menu (three dots) > Settings > Privacy and security > VPN Settings.
- Toggle the VPN on from there.

If after these steps the issue persists, we can try reinstalling the Avast Secure Browser or checking if your Avast SecureLine VPN or other antivirus software might be conflicting.

Would you like me to guide you through any of these steps now? Also, please share more details about when exactly the browser closes, so I can assist better.

Are you comfortable trying these?