Norton Anti Virus 2004

[jmh276]

I have norton anti virus 2004 as part of the systemworks package to protect my computer which runs windows xp.
It sometime catches trojan attacks and deletes them.
However, every month or so my Internet explorer has a different default search engine/homepage etc.When I run the virus scan it tells me I have a number of spyware/adware programmes.It cant delete most of them so I have to do it manually.
Does anybody know why the internet security/antivirus software doesnt stop them infecting my computer.I have checked all settings and keep the definitons up to date but I still get infected.
Any advice would be appreciated

[pip22]

You've been hit by what's known as a 'Browser Hijack', which changes your browser's start-page to something else you didn't ask for. Antivirus programs can't remove them because thay are not viruses. Some of these hijackers are vewry stubborn critters. You think it's gone but when you next restart it's back again. That's because they put entries into the Windows Registry which need to identified and removed. You should download 'Spybot - Search and Destroy' . Update it then run it. Remove all it flags up. But you may also need to download and run a specialist hijack remover called 'Hijack This'. Run it, create a log file with it and post it on one of the forums where they read your log file and tell you what to remove. I don't think this forum provides that service, but I could be wrong. Understanding the log-file and knowing what shouldn't be there is a specialist job, so be sure to post it on a forum which can deal with it.