Which type and brand of router ?
Several computer models include two specific hardware components that enable the device to connect to the Internet: an Ethernet adapter card and a wireless adapter card. These cards are also called network adapters, or network cards. If your computer suffers from an intermittent Internet connection, and your Internet service provider (ISP) doesn't detect any issues on their end, you can update the software for the network adapters to resolve the intermittent connection. If updating the adapters doesn't resolve the problem, reset the transmission control protocol/internet protocol (TCP/IP) and Winsock to restore the connection.
Click "Start." Click "Control Panel." Type "manager" into the search bar. Click "Device Manager." Expand "Network Adapters." You should have one wired adapter and one wireless adapter, if the computer has wireless support. Double-click each adapter. Write down the name of the adapter, as well as the manufacturer's name. Go to the computer or network adapter manufacturer's website if you installed the adapter to the computer yourself. If the adapter came with the computer, go to the computer manufacturer's site instead. Select the "Support," "Download" or "Drivers" page. Look up your computer or adapter model. Select your operating system. Click the "Network," "LAN" or "Internet" category. Download the latest adapters for your network devices. Click "Save" or "Save As" and save the files to your desktop. Return to Device Manager. Right-click the first device. Click "Update Driver Software." Click "Browse My Computer for Driver Software." Follow the remaining on-screen directions to reinstall the device driver. Repeat the process for the other network device. Restart the computer after reinstalling the drivers to repair the intermittent Internet issue. Reset IP
Log in to Windows as an administrator. Hold down the Ctrl, Shift and Esc keys to open Windows Task Manager. Click "File." Click "New Task." Type "cmd" and press Enter to open Command Prompt. Type "netsh int IP reset c:\resetlog.txt" into the Command Prompt. Press Enter to rewrite the TCP/IP registry keys. Enter "netsh winsock reset" into the command-line prompt. Press Enter to reset the Winsock catalog and resolve the intermittent Internet problem.
How to reset Internet Protocol (TCP/IP) http://support.microsoft.com/kb/299357