Internet gradually getting slower, any help?

I recently got Internet installed in my own room, and for the first couple of days, the speed was fine, great actually, considering the fact that I'm paying only a little over 200 pesos for it (approximately $20). Then suddenly, after a little over 2 weeks, the browsing speed started slowing down considerably, to the point that right now, I had to reload Yahoo Answers around 7 times just to be able to post this question, because I kept getting "the connection had timed out".

Odd thing is, download speed stayed the same. I managed to download some stuff from Megaupload and it was speedy as usual. Only browsing was affected, apparently.

Now, I'm pretty sure it's the computer itself, because I connected the Ethernet cable to my dad's laptop and the speed was just as good as the first few days on my computer. Also, I've been surfing (ohmyGod,youdontknowhowmuchofapainitwas) through the Internet trying to find common solutions, and I tried every single one. I scanned for viruses and Spyware with AVG and SuperAntiSpywarePro, deleted Cache, Temp files, used System Restore to a point before the problem, cleaned out all of my useless files (the speed's fine for anything except browsing anyway), used CCleaner, reset my router, used Windows' Diagnose and Repair connection, and even tried several other browsers. Nothing worked.

Now, please, I went through too much trouble to even post this question, so, do you guys have any other suggestions or an idea of anything else I could try?

Computer's a Toshiba Satellite laptop, with Windows Vista, 2GB of RAM. My ISP is a Mexican company, Megacable, which I don't think you could do much with that info. Browser is Firefox, I recently updated to 7.0.1, which at first I thought was the cause of the problem, before I tried Google Chrome and Opera and they didn't work either. I do use BitTorrent, but I stopped using it to try and resolve the internet problem.