I need help URGENTLY!?

my dad is gonna go mad at me if i do anything to the computer which i dont know about so i really need help, heres the details:

About 2-3 days ago, i went on the internet and it was running slow, browsing pages really slow and i couldnt figure out the problem til my dad said it's proberly the signal. So i didnt think much of it. But then, it's been happening afterwards the next day and the next, I've scanned my PC with avast! anti-virus, CCleaner, spybot search and destroy and rootkit remover, found none of them things, and it's still browsing so slow. My msn wont even sign me in because it says service is temporarily unavailble, check that you're connected to the internet and try again, and checking google, it took about 1-2 minutes to get on there. My pc has no viruses or cookies or spyware or rootkits. So what is going on?? my computer is also going abit slow aswel, i really need to do something about this before my dad starts to go ballistic, i'm no computer geek so.. please help?

I need help URGENTLY!?symatec

If this is a MS Windows OS then first thing is check your system resources available. To do this close all programs and applications you are using then hit the "ctrl + Alt + dlt" keys at the same time then click performance tab. Check the memory usage against system memory installed and the CPU usage. If you are using more memory than is installed or the CPU usage is over 30% with all apps closed then something is eating up your resources.