First I would check to make sure all the necessary ports are still open on your firewall and router. Sometimes an update or another program can modify port settings.
This issue may occur if a third party program has corrupted the Winsock catalog on the computer.
Refer this KB article and follow the steps to reset TCP/IP stack.
How to reset Internet Protocol (TCP/IP)
The article is for Vista, but it also applies for Windows 7.
If that doesn’t help, then do a clean boot.
Refer this link for more information:
Steps to restore your Normal startup:
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 Normal Startup.
3. When you are prompted, click Restart.
Click Start, type services.msc in the search box and press enter.
Set the following services to
Startup Type: Automatic
Service Status: Started
-Background Intelligent Transfer Service
-SSDP Discovery Service
-Universal Plus and Play Device Host
Once the above are set to started and automatic, restart the computer.