Viruses screwed my computer up. How to fix?

i have windows xp. we scanned with spybot malwarebytes smitfraud ccleaner ad aware prob.



Ok so i got some viruses a few days ago and my brother appears to have removed or eliminated them but

i think they screwed stuff up while on my computer.

I keep getting those svchost.exe errors about memory except i havent been able to find matching numbers

online for mine. I got a code here its.... 0x608c6de0 but i have had others.

Also, i cannot connect to the internet as i cannot even find the usual place where i try to connect.

Also, when i click on FireFox it says in the address bar zitodns.com or some link and i cant remove it.

And Explorer just doesnt connect nor would it let me change firewall settings.Viruses screwed my computer up. How to fix?
Install Avira Free-AV and run it in safe mode.