Windows unable to connect to WiFi / unable to connect wireless connection in windows 7 /XP /Vista

This issue occurs because of a difference in design between Windows 7 and Microsoft Windows XP Service Pack 2 (SP2). Specifically, in Windows XP SP2, the BROADCAST flag in DHCP discovery packets is set to 0 (disabled). In Windows Vista, the BROADCAST flag in DHCP discovery packets is not disabled. Therefore, some routers and some non-Microsoft DHCP servers cannot process the DHCP discovery packets.

To resolve this issue, disable the DHCP BROADCAST flag in Windows 7. To do this, follow these steps:

1.    Click Start, type regedit in the Start Search box, and then click regedit in the Programs list.
If you are prompted for an administrator password or for confirmation, type your password, or click Continue.

2.    Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{GUID}
In this registry path, click the (GUID) subkey that corresponds to the network adapter that is connected to the network.

3.    On the Edit menu, point to New, and then click DWORD (32-bit) Value.

4.    In the New Value #1 box, type DhcpConnEnableBcastFlagToggle, and then press ENTER.

5.    Right-click DhcpConnEnableBcastFlagToggle, and then click Modify.

6.    In the Value data box, type 1, and then click OK.

7.    Close Registry Editor.




By setting this registry key to 1, Windows 7 will first try to obtain an IP address by using the BROADCAST flag in DHCP Discover packets. If that fails, it will try to obtain an IP address without using the BROADCAST flag in DHCP Discover packets.

For more information visit the following link:
http://support.microsoft.com/kb/928233

Note: This link is also applicable with Windows 7.

Method 2:

Disable the IP Helper service:

1. Hold the Windows key and type R, enter "services.msc" (without the quotes) and press Enter
2. Scroll down to the IP Helper service, right click on it and select Properties
3. In the dropdown box that says "Automatic" or "Manual", set it to Disabled and then click on "Apply"
4. Then click on "Stop" to stop the service from running in the current session
5. Click OK to exit the dialog

Method 3:

Disable IPv6:

1. Hold the Windows key and type R, enter "ncpa.cpl" (without the quotes) and press Enter.
2. Right click on each network connection and select "Properties".
3. Remove the checkmark from the box next to "Internet Protocol Version 6 (TCP/IPv6).
4. Click OK to exit the dialog.

Method 4: Disable the firewall settings of the computer.

Windows Firewall is on by default in this version of Windows. To make sure it hasn't been turned off, follow these steps:
1.    Open Windows Firewall by clicking the Start button, and then clicking Control Panel. In the search box, type firewall, and then click Windows Firewall.
2.    In the left pane, click Turn Windows Firewall on or off. If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
3.    Below each network location type, click Turn off Windows Firewall, and then click OK. We recommend that you turn on the firewall for all network location types.