I am unable to access sites such as gmail, hotmail,facebook and orkut on my PC. The main page of these sites appears on screen, but as soon as i enter the username and passwords, the page displaying "the page cannot be displayed" appears. The problem has been since 2 mnths now. i could earlier access these sites without a problem. I also cannot download anything through aires.

This is a symptom of a malware infection. There is some kind of program that has been installed on your computer that is hijacking your browser.
Go here: MBAM Antimalware and attempt to download their free antimalware software. If you cannot get the download to work, you will have to do it from another computer and burn the file to a cd to install it on your system. Install the software and run a full scan of your computer and choose to fix any problems it finds. Do this without rebooting the computer after installation.
If you have trouble installing the program, follow the instructions at the bottom of the link page and someone will help to guide you through the process, or you can post a comment to this post and I'll try to walk you through it.
You need to address this immediately! Your personal information could be compromised by continuing to use your computer without correcting this problem...Please act now.