Right click the task bar icon and select 'On-Access Protection Control' Look for 'Web Shield'. Try hitting pause and see if things speed up. If that works either try customising or terminate it. If you only ever access the reputable upright sites like PCA or the BBC it's probably not necessary. If you ever go random browsing off the beaten track, leave it on.
I don`t think your actions of offloading Norton and installing AVG have much to do with your problem. If you`re using Win XP then go to Start>All Programmes .Accessories> System Tools> Disc Cleanup and follow it to saying `yes` to clearing everything. Alternatively go to Control Panel> Internet Options and then Delete Cookies followed by Delete Files. There will be no harm done
I too found Avast a bit of a performance downer (though less than Norton) - noticeable particularly on systems short on RAM.
If you want the best AV at detecting viruses, and at the same time the AV with the least effect on performance, and the fastest scanner by far - then you need NOD32 click here - free trial, but then you pay an annual fee.
NOD32 is often recommended on gaming sites due to it's minimum effect on performance.
The latest version certified against Spyware and Trojans too.
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