There is a bug running around infecting Java, it has been posted on the net for several days. ORACLE announced that they were working on a fix for this issue. You may be one of those who have been affected by this nasty. As I recall the basic fix was to disable Java from control panel and/or the browser. Another thing I would take a look at would be the home page setting in control panel to see if your default homepage has been altered, as in a redirect. It could be something entirely different but from the sound of it and what I saw on the net news a few days ago it sure smacks of the Java bug.
So yesterday I wanted to search something on google, and when i hit "search" the browser window got all blank(I'm using opera ).
Then i tried with google chrome, but this time it wrote "webpage not avilable". Same thing happens with Facebook and Youtube.
Today i downloaded Avast, and when i tried enterting those sites again, Avast blocked them, giving me this text:
"Object: http://www.google.com
Infection: JS:ScriptIP-inf [Trj]
Procces: where my chrome is intallled."
Same on opera and explorer. Cleared caches, did not work.
Same thing happens with Gmail, or other google services.
I have wi-fi in my house, and with my android phone i can enter any of those sites without problem, so im guessing it's my computer's problem. Any advices?

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