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What to do if my computer is infected by a virus or a malware?
Every time I change the Home page in Firefox or IE it returns back to http://www.smartwebsearch.net ?
What is wrong with my PC? Is it a Trojan or spyware?
Yes it's the result of a trojan. There is a post here that might be of use. Basically you may be able to remove it with something like MalwareBytes. Unfortunately if the problem returns or you are unable to run MalwareBytes then there could be an underlying rootkit in which case you'll need something like rootrepeal. Did you have any anti-virus software installed before this happened?