Sometimes it is possible that the network settings on Windows7 are in confusion. For example, after a deinstallation of Firewall software that often changes the Winsock settings. 

To solve this problem, it will be necessary to reset the network settings. To do that, there is a command line tool called netsh.

Please note! The following steps should only be done by advanced users!

Go to the Start button at the bottom of the left side.
Enter "cmd" in the field "search programs/files"
You will then get a list, similar to the following one:

Then click "cmd.exe" with the right mouse button. In the shown menu choose "Run as administrator". 
If once a password for the administrator was chosen, you will need it now.

The window to enter the command line (cmd.exe) will open; this window is also known as the DOS window.
Then you need to enter the commands "netsh winsocks reset" and "netsh int ip reset", each followed by the enter button.

You should get a message that the computer needs to be restarted.
After the restart the network should work flawless again. 
If not, there are other/more serious problems.