Hi PrakashRamalingam,
Welcome to Microsoft Answers community.
a. since when are you facing this issue?
b. Did you make any changes to your computer recently?
Disable security software and check if the problem persists.
I would also suggest you check if the problem persists in safe mode with networking. If there is no problem in safe mode with networking then perform clean boot.
Method 1:
To use a Safe mode with networking option, follow these steps:
1. Restart your computer and start pressing the F8 key on your keyboard. On a computer that is configured for booting to multiple operating systems, you can press the F8 key when the Boot Menu appears.
2. Use the arrow keys to choose Safe mode with networking in Windows Advanced Options menu appears, and then press ENTER.
Method 2:
****Starting your computer by using a minimal set of drivers and startup programs so that you can determine whether a background program is interfering with your game or program. This kind of startup is known as a "Clean boot."
To perform a clean boot on a computer, follow these steps.
1. Click Start, type msconfig in the Start Search box, and then press ENTER.
If you are prompted for an administrator password or for a confirmation, type the password, or click Continue.
2. On the General tab, click Selective Startup.
3. Under Selective Startup, click to clear the Load Startup Items check box.
4. Click the Services tab, click to select the Hide All Microsoft Services check box, and then click Disable All.
5. Click OK.
6. When you are prompted, click Restart.
7. After the computer starts, check whether the problem is resolved.
Please monitor the system in the Clean Boot environment. If the problem does not occur, it indicates that the problem is related to one application or service we have disabled. You may use the MSCONFIG tool again to re-enable the disabled item one by one to find out the culprit.
If your issue is resolved, follow the How to Determine What is Causing the Problem section in the KB article to narrow down the exact source.
http://support.microsoft.com/kb/331796 . Also, see the section on how return your computer to a Normal startup mode
If the above methods were not useful in resolving you issue then, try the below method.
Method 3: Reset TCP/IP stack.
To reset the TCP/IP stack go to this article and either click on "Fix it for me" or follow the instructions to fix it yourself: http://support.microsoft.com/kb/299357
I would also suggest you to update wireless adapter driver.
Steps to update wireless adapter driver:
1. Click on start button.
2. In the search box type devmgmt.msc and then press enter.
3. Select the <wireless adapter> device and right click on it
4. Now select properties.
5. In the properties window, under Driver tab, click on Update Driver button.
6. After the installing the updates restart the computer.
For more information visit http://windows.microsoft.com/en-US/windows-vista/Finding-drivers-What-to-do-when-your-hardware-wont-work-with-Windows-Vista
Also run an online malware/virus scan on the computer.
http://onecare.live.com/site/en-US/default.htm
Hope this information is helpful.
Let me know if this worked.
Thanks and Regards:
Umesh P - Microsoft Support