Can't load certain webpages/Can't upload files/etc.

I've been having problems with my computer for about a month now. I can load Google's homepage, but cannot do searches without a proxy. I can't load gmail, hotmail, blogspot, can't upload any files to mediafire or any other website, can't upload avatars or upload pictures to imageshack, tinypic, etc. Most other sites will work, but the internet has become considerably slower. Occasionally this will stop and I will be able to upload things and do Google searches, but this will revert back usually within an hour or so. I am able to download any file, any filesize without problems however. No other computers on my network are affected.

I've resetted my modem, restarted, attempted to reinstall windows but the computer will not go past the "loading setup" screen (I don't know if this is part of the problem or not), tried a different browser (Usually use firefox, but tried IE and same problem; even downloaded Opera and tried it, same result). I use AVG and it picked up nothing so I downloaded BitDefender and it only picked up spyware and trojans unrelated to this problem. MSN messenger works completely fine and logs in without problem; Last.fm continues to send data, etc. Filesharing through MSN and AIM, etc, are no problem. Filesharing programs originally worked fine, but seemed to have been affected recently.

I did quite a bit of research on it and I think I might have a Vundo trojan. I will occasionally have ads from websites (such as Google ads) replaced with an ad that says "you should scan your computer etc etc" and even replaces some normal thumbnails of images on image boards with these ads. I took a screencap of one of them, you can see it here: link. Also, at random times my browser will open displaying a page with similar information, saying "you have malware etc etc. buy our program scan your computer", but I don't know if it is surely related or not. I've tried VundoFix in case I did have Vundo, the first time it deleted 4 files and the second time about 8, but with the usual result nothing had improved. Also, please tell me whether to post my HijackThis log.

Thanks in advance and sorry for the trouble. Also, please excuse me if this ends up being the wrong forum.