What is wrong with my internet?

Okay, Here is the deal with my computer.

I downloaded Mega Upload before.
Yes, I deleted completely from my FireFox (Deleted from Add/Remove Program and "Add-On" window). However It gives me the same problem, just without the DLL Error or something like that.

I get on to some sites, it opens up, like GWG. But other sites I visit, they don't open at all. It sometimes occur randomly.

My homepage, Daum, sometimes it opens, sometimes it gives "Problem loading page".

I reseted my modem for around fifth time, and still doesn't work.

My GW works, but connecting takes forever.

Anyone have suggestion?

Is it my modem's fault? Mine? or AT&T's?

Thanks for helping

-Chiyo

Do virus scan. Do a rootkit scan. Do a malware/adware scan.

Not necessarily in that order and not necessarily only once.