If you check my signature below my post you will see one of the systems that I’m using is winXP Pro SP3 and I have the latest avast free version 17.5.2303 and no Issue with the avastUI loading.

Check that you have Terminal Services running, open the Windows Services - Windows key + R - now type services.msc and enter. Now you should be able to check the Services for Terminal Services running.

Whilst mine is set to Manual it is running - if yours isn’t running and set to manual - try setting it to Automatic and restart your computer.