Hijacked computer - any suggestion(s)?

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In the past few months the computer that I am using has been hijacked for more than 10 times. Even though I know a lot about computers I do not know everything. That is why I am posting this discussion in the hope there are others out there who are willing to share their knowledge and/or experiences. It seems that there are two types of hijacking. The end result is my computer was used, presumably for unsavoury activities. Signs included slow Internet access. Computers from China, Korea, France, Nigeria, and the USA have managed to take over control Now I have been forbidden from visiting sites which I used to visit before. Usually I restarted the computer. This seemed to work for a few days before my PC was again overrun. I am using AVG 8.0 free edition and Zone Alarm Firewall. I still lost the battle. Has anyone experienced similar attacks? What did you do to repel them? Any specific software to prevent undetected intrusions? Any reasonable suggestion would greatly be appreciated.