Very slow browsing, streaming and almost no downloads (unknown network error)

I have a broadband connection of 512Kbps in my Windows 7 PC. Since yesterday I am unable to browse anything on the internet properly. Some webpages take a lot of time to load (>3mins) and a few webpages don't load at all (it keeps on loading but nothing shows up on the screen. The screen's blank with no error message). Even Youtube doesn't stream quickly. It takes almost 5-6 mins for even a 144p video to load and it buffers a million times! Also I am unable to download stuff from the internet. Like I was downloading TCP optimizer today and it stops at 80-90% everytime and later shows - "unknown network error" in chrome. I have tried Mozilla and IE as well but in vain.
Surprisingly Facebook, gmail and a few other websites work like a charm without any major hassle. Even utorrent works perfectly well.
I am unable to browse webpages (educational sites, tech sites, gaming sites) or download through any browser, stream videos online. I have tried all the browsers available (Chrome, IE, Firefox). Tried uninstalling reinstalling the anti-virus, switched off windows firewall, tried networking in safe mode, used RegCure Pro, and also C cleaner. Nothing worked.
Please provide a good solution to my problem.
Thank You.

Computer Info:

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Ultimate, 32 bit
Processor: AMD Athlon(tm) Processor LE-1660, x64 Family 15 Model 127 Stepping 2
Processor Count: 1
RAM: 1918 Mb
Graphics Card: NVIDIA GeForce 6150SE nForce 430, 128 Mb
Hard Drives: C: Total - 49999 MB, Free - 23832 MB; D: Total - 99995 MB, Free - 3521 MB; E: Total - 155237 MB, Free - 15913 MB;
Motherboard: PEGATRON CORPORATION, NARRA5
Antivirus: avast! Antivirus, Updated and Enabled