Help with pop-ups in XP R2

Hi.
Sorry if i post something that already have been answered, but i cant find an answer searching here.

I have a problem: entering any site, i get constant popups asking me for user/passwd to a lot of sites: googlesyndication, edge-quantserve (this one entering here), and a lot more. I simply cancel it and the page seem to load ok, but sometimes without parts of it.
Ive used cccleaner and adware, doesnt seem to help. It happens with Explorer and Firefox.

Can anyone help me? Please count with the fact that im quite new in these things and dont know much of configuring or tweaking with the pc.
Please tell me what information about the configuration you need, and with wich tool i can get it.

Thank you all in advance.

 

Sorry, its me again, forgot to say, i would not be able to see the forum in at least 4 or 5 hours. Thank you all again.