Thanks for the answers. I understand that, due to Anti Virus or Firewall, these problems can occur. However, I completely switched off my anti virus and at one time even uninstalled the anti virus software. Although, with the same anti virus setting I was able to login before this incident. So even anti virus in not the case.
Now the only thing remained to check is that whether my ISP is blocking certain ports. Can anyone tell me that what are the most widely used ports for SL? Thanks for your support and answers.