system
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A bad sector means your HD is dieing! I had it once after i just bought my “new” (now old) computer back in 1999. 1 month later I had a bad sector and 1 day later I got many more! 
At one point I got 5 GB of bad sectors! 
A virus or AV can’t damage hardware,…only software!
The fact with hardware is,…if it has to break,…it will,…today or tomorrow 
When I bought my new computer in november 2004 the firs day i got it, my graphical card started smoking! Off course at first you blame the game you where playing at the time. And then I got a replacement graphical card and I played the game again,…a bit uncertain,…and nothing happened! ;D
So,…I think you may safely install Avast! on your new system,…you will be more then satisfied,…especially with v4.6 coming up!