system
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I had a similar problem. My experience might possibly help you.
I firstly used CCleaner to clean out old temp files and old prefetch entries, also old directories under program files, program data and user data from any previous security software, then rebooted.
Then i used the CC registry cleaner and rebooted again.
I then installed V7 of Avast rather than v8. to avoid automatic upgrade need to do this with internet diconnected. When installed, I selected the option for the program not to update itself automatically (I do not want V8 due to problems and GUI). Also I unticked Webrep and Sitecorrect under browser protection (I know what I am doing, but others may need / want these enabled).
Then rebooted and reconnected internet cable to PC. All running smoothly with no lag / delay in browsing. Hope this helps.