Screen is blank after installing Avast!

I am a long time avast user. I like the program but I had a friend to install avast on her computer. She states that after installing it asked her to reboot the system in which she did.
After it came back up the screen black. She started several times with same results. All lights are on and you can hear the Windows shutdown music tone.

She has a Inspiron 600 uma Dell computer (laptop)
Windows XP Home Edition
Celeron 3.5 1.3 GHz
1 MB cache
400 MHz fsb
512 MB shared DDR
60 G hard drive
Graphics- excelerator 900 graphic card.

Can someone help?

Hello,

Did her try the safe mode?

Do you know how to enter in safe mode?

Thanks for your time,

Elminster

Does she has any other antivirus installed in the same machine?