free home antivirus

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I'm using prgm version 2015.10.0.2218 on Vista, and I can't get the file shield to turn on. I tried clicking on fix now and by going in to turn it on myself. 
Also, it says new program version is available, but there's no new updates for the program or virus definitions. 
How do I fix these issues?

Thanks! 

Z

[Asyn]

Repair Avast:
1. Control Panel -> Add/Remove programs -> Avast
2. Click on 'Repair'.
3. Follow instructions.
4. Reboot.