system
4
Its probabily:
1-Your Hard Drive is Damaged by Bad Block
2-Aplication Conflicts.
Please, turn ON your computer, in the black screen about your computer and BIOS, hit F8 key several times until the security mode option appear.
In the Security Mode, run avast scan.
If your computer shutdown, is a possible Hard Drive Damaged, and your computer not shutdown, possible programs conflicts in the normal mode with other antivirus, antispywares, etc.
Goodbye!