While downloading from adobe.com, Microsoft.com, nvidia.com my download speed is 500 kb/s but while from other websites like torrents and other my download speed is only 20kb/s. Why is it?? please reply me fast.
As you've found the download speeds from different sites are different. That's why when you download something enormous (like a Linux package) they offer a choice of download sources.
Torrents are links with other users (thus the great danger of picking up malware). As most are on ADSL (the A stands for Asymetric) the upload speed from their machines will be considerably lower than the download speed.