Restart in safe mode so you can find the problem or at least uninstall or reinstall Avast.
To use the F8 method
Use this method only if Windows XP is the only operating system installed on your computer.
1. Start Windows, or if it is running, shut Windows down, and then turn off the computer.
2. Restart the computer. The computer begins processing a set of instructions known as the Basic Input/Output System (BIOS). What is displayed depends on the BIOS manufacturer. Some computers display a progress bar that refers to the word BIOS, while others may not display any indication that this process is happening.
3. As soon as the BIOS has finished loading, begin tapping the F8 key on your keyboard. Continue to do so until the Windows Advanced Options menu appears. If you begin tapping the F8 key too soon, some computers display a "keyboard error" message. To resolve this, restart the computer and try again.
4. Using the arrow keys on the keyboard, scroll to and select the Safe mode menu item, and then press Enter.
Once in safe mode, you screen shouldn't be frozen.