Avast Antivirus error after it was updated.

Hello,
My mom just fallen into a "big problem" with her computer. This is about a desktop of old Dell Dimension 4600 running in XP Home.
Mom runs Avast Antivirus software, and this morning it was declared to update the application. She pressed where she told to click. After the update process, Avast told her to restart her computer, and the computer goes through the restart process. All was going normally, but suddenly, the system just freezes. Her desktop wallpaper and desktop icons are (I think) present. But the taskbar / start menu / systray icons is not visible. At the same time she receives an error message says:
"Unable to start applications -2147023174," with a pop up that requires to restart the computer. She can only restart it via the hardware button. She repeatedly doing it, but each time she faces the same problem.
I can help her at some point, but she's in a hurry and the internet connection on her system is slow, so I want to do research as much as I can before I go.
Any approach to solve this problem would be appreciated. Thanks.
